Math and shit

"Wheelerguy" (wheelerguy)
06/15/2017 at 09:33 • Filed to: Math

So my little brother has a math problem, and one of them is this:

1/7+1/21=?

He insists on dividing the 21 with the 7, which is massively contrary to what I was taught, which is to find the least common multiple and add from there.

Full list of problems:

1.) 1 1/2 + 1 1/2 =? (The answer is 3.)

2.) 2 1/6 + 1/3 =?

3.) 1 1/8 + 1/6 =?

4.) 1/7 + 1/21 =?

5.) 1/9 + 1 1/18 =?

Show your solution, aye?

DISCUSSION (43)

Nibby > Wheelerguy
06/15/2017 at 09:36

the answer is

tree fiddy

Flavien Vidal > Wheelerguy
06/15/2017 at 09:37

4/21... Multiplications and divisions always have priority on additions...

Ash78, voting early and often > Wheelerguy
06/15/2017 at 09:38

Mentally, I just convert the 1/7 to 3/21 and add across the numerators. Least Common Denominator is still a thing, right?

PartyPooper2012 > Wheelerguy
06/15/2017 at 09:39

lol. You need a least common denominator. so you want to convert 1/7 to have 21 in denominator. so 3/3 * 1/7 = 3/21
now you can add 3/21 + 1/21 = 4/21 which is the answer.

pip bip - choose Corrour > Wheelerguy
06/15/2017 at 09:40

q2 = 1/2?

q3 = 1/2?

q4 = 0.2?

q5 = 0.16666?

PartyPooper2012 > Wheelerguy
06/15/2017 at 09:40

lol your first question - answer is 2 by the way. 1/2 + 1 1/2 is 2.

Roundbadge > Wheelerguy
06/15/2017 at 09:41

Denominators always have to match...so convert to lowest common denominator.

2. 2 1/2 (1/3 = 2/6. 2/6+1/6 = 3/6 = 1/2)
3. 1 7/24 (1/8 = 3/24, 1/6 = 4/24. 3/24 + 4/24 = 7/24)
4. 4/21 (1/7 = 3/21. 3/21 + 1/21 = 4/21)
5. 1 1/6 (1/9 = 2/18 . 2/18 + 1/18 = 3/18 = 1/6)

Honeybunchesofgoats > Wheelerguy
06/15/2017 at 09:42

Kinja'd!!!

PartyPooper2012 > Wheelerguy
06/15/2017 at 09:42

by the way, you can check your answers in google by typing in the problem for instance 1/7 + 1/21 = .1904761904761905 which is same as 4/21

Wheelerguy > PartyPooper2012
06/15/2017 at 09:43

Wait no both of them are 1 1/2.

Nibby > Flavien Vidal
06/15/2017 at 09:43

PLEASE EXCUSE MY DEAR AUNT SALLY

diplodicus > Flavien Vidal
06/15/2017 at 09:45

This is correct, you would multiple 1/7 by 3 to get a common denominator 3/21 + 1/21=4/21. You get the same answer if you use order of operations and divide 1/7, divide 1/21 and add the decimals together.

facw > Wheelerguy
06/15/2017 at 09:45

As others said, you multiply top and bottom of 1/7 by 3 to get 3/21, which you can then add to get 4/21. I’m not sure what you mean by dividing the 21 with 7 or why you’d want to. You can do:

21(1/7+1/21) = 21x

3 + 1 = 21x

4 = 21x

4/21=x

but that’s really more or less the same as above. You could do that with 7 as well, but then you still have a fraction to deal with:

7(1/7+1/21) = 7x

1+1/3 = 7x

3/3+1/3 = 7x

4/3 = 7x

4/21=x

But why?

PartyPooper2012 > Wheelerguy
06/15/2017 at 09:46

lol ok. cuz you had a list that started with 1. I figured you mistook the number 1 problem followed by half to be be 1 1/2... anyway

ttyymmnn > PartyPooper2012
06/15/2017 at 09:49

The correct answer is 2/10.5

PartyPooper2012 > ttyymmnn
06/15/2017 at 09:51

gasp! I thought the answer was 42.... Are you from the future past? Past future?

Smallbear wants a modern Syclone, local Maple Leafs spammer > Wheelerguy
06/15/2017 at 09:52

He’s not wrong, but that’s not a step in the final solution, really, it’s a check-step. Dividing 21 by seven tells you that it is evenly divisible, and by what (3). From there you know what you need to multiply both numerator and denominator by to arrive at the common denominator of 21. Since you don’t need to multiply 7 and 21 to finda common denominator why bother?

(1*3)/(7*3)=3/21

3/21+1/21=4/21

4/21=0.190476190476190476190476blahblahblah so we stick with 4/21.

What he’s doing is putting on paper what you or I know in our heads.

For the rest... well personally, I hate mixed numbers 9 out of ten times... I find it easiest to turn them into improper fractions.

1. You answered it.

2. 2 1/6+1/3, 13/6+1/3, 13/6+2/6, 15/6... or 2 3/6 if you prefer. Alternately, you know that 1/3=2/6, just ignore the “2" in “2 1/6" for a moment, add 1/6+2/6, and put the 2 back where it belongs.

3. 1 1/8=9/8, 9/8+1/6, (9*6)/(8*6)+(1*8)/(6*8), 54/48+8/48, 62/48, 31/24, 31-24=7 so 1 7/24.

etc.

jariten1781 > PartyPooper2012
06/15/2017 at 09:53

Or just use WolframAlfa and it will give you the fraction directly.

PartyPooper2012 > jariten1781
06/15/2017 at 09:55

you could... but you learn nothing. Same as if you googled it.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Honeybunchesofgoats
06/15/2017 at 09:56

Fractions are good

jariten1781 > PartyPooper2012
06/15/2017 at 09:58

If you’re actually trying to learn the mechanics you can click for the step-by-step solution and it will walk you through establishing a common denominator/adding/simplifying; if you’re checking the answer it just saves a step of having to divide both sides since it gives it directly.

Agrajag > Wheelerguy
06/15/2017 at 09:59

PartyPooper2012 > jariten1781
06/15/2017 at 09:59

yup. there are many places where you can plug in math and get the answer and/or be shown work. I was just suggesting that the easy peasy way is to plug it into google. No applications needed.

Vítor > Wheelerguy
06/15/2017 at 10:01

Question 1: 1 1/2 + 1 1/2 = 3/2 + 3/2 = 6/2 = 3

Question 2: 2 1/6 + 1/3 = 13/6 + 2/6 = 15/6 = 5/2 or 2 1/2

Question 3: 1 1/8 + 1/6 = 27/24 + 4/24 = 31/24 or 1 7/24

Question 4: 1/7+1/21 = 3/21+1/21= 4/21

Question 5: 1/9 + 1 1/18 = 2/18 + 19/18 = 21/18 = 7/6 or 1 1/6

McMike > Honeybunchesofgoats
06/15/2017 at 10:17

You’re exempt from the “penmanship and shit” installment when it comes out.

themanwithsauce - has as many vehicles as job titles > Honeybunchesofgoats
06/15/2017 at 10:17

Meanwhile at my job, scientific notation and algebra are pretty much used every day.....

And getting good with fractions helps with dimensional analysis. Instead of finding the lowest common denominator, you find the lowest common units of measure to convert to.

ttyymmnn > PartyPooper2012
06/15/2017 at 10:20

Yes. Do you have your towel?

ttyymmnn > Wheelerguy
06/15/2017 at 10:21

I was told there would be no math. My son is going into HS in the fall. I struggled with his MS math, and I am so fucked next year. Fortunately, his uncle is a math teacher. So I use my Phone a Friend lifeline.

themanwithsauce - has as many vehicles as job titles > Wheelerguy
06/15/2017 at 10:24

First, find the LCD. 21>7 so try 21/7.

21/7 = 3. Excellent. Halfway to Miller Time.

Multiply both the numerator and denominator by the LCD.

1/7 X 3/3 = 3/21.

Both fractions have the same denominator so now you can add the numerators straight across.

3/21 + 1/21 = (3+1)/21 = 4/21

Is that what they want?

Honeybunchesofgoats > McMike
06/15/2017 at 10:25

My mom made me learn cursive in kindergarten, “because it’s going to be essential when you grow up.”

She’s going in a home the first chance I get.

PartyPooper2012 > ttyymmnn
06/15/2017 at 10:28

I air dry things.

McMike > themanwithsauce - has as many vehicles as job titles
06/15/2017 at 10:51

I use math every day when Mrs McMike needs help figuring out how many fluid ounces are in a pound when she’s reading a recipe.

Future next gen S2000 owner > Wheelerguy
06/15/2017 at 11:07

1.) 3 2.)2 1/2 3.)1 7/24 4.)4/21 5.) 1 1/6

Did these in my head. So I may need someone to check my math.

merged-5876237249235911857-hrw8uc > McMike
06/15/2017 at 11:24

16oz to a lb. that ones simple.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Honeybunchesofgoats
06/15/2017 at 11:57

I forget cursive pretty much as fast as I learned it.

I retained enough to sign my name... if I go really slow. Otherwise it starts as my name and finishes as random scribble

Meatcoma > Wheelerguy
06/15/2017 at 13:41

Lowest common denominator is 21 since 7 goes evenly into 21, make them both a fraction of 21.

1/7 * 3 = 3/21.

3/21 + 1/21 = 4/21.

McMike > merged-5876237249235911857-hrw8uc
06/15/2017 at 14:11

Fluid ounces. One is volume one is weight.

merged-5876237249235911857-hrw8uc > McMike
06/15/2017 at 14:38

Understood, but it’s typical that 16oz = 1lb.

McMike > merged-5876237249235911857-hrw8uc
06/15/2017 at 14:48

This is the same conversation I have at home.

You’re not wrong sweetheart, 16 oz is exactly 1lb.

But 16 fluid ounces is not the same thing as 16 ounces. You use a scale for ounces, but you need to use measuring cups for fluid ounces. Does the recipe say “fl oz” or just “oz?”

PartyPooper2012 > diplodicus
06/15/2017 at 15:02

Just being a party pooper, but if you multiply 1/7 by 3, you would get 3/7.

3/1 * 1/7 = 3/7.

You want to multiply it by a fraction that amounts to 1 as multiplying by 1 doesn’t change anything.

3/3 = 1

so you want to multiply 3/3 * 1/7. This would then give you a 3/21

diplodicus > PartyPooper2012
06/15/2017 at 15:18

You’re right it’s been too long since I’ve really done this and didn’t catch that.

merged-5876237249235911857-hrw8uc > McMike
06/15/2017 at 21:32

Lol

NJAnon > Wheelerguy
06/15/2017 at 22:59

fractions....aka the beginning of the end.

Good luck to your brother though. math literally causes nervous breakdowns for me. Im still talking to someone that is qualified.