Fraction help needed!!!

[tiggerlover]

DS has a math problem involving fractions, and although I know what the answer is, for the life of me I can't figure out how to show DS the solution.

If you have a line segment that is 2 7/8" long and you erase 1/2", how long will your line segment be?

The answer is 2 3/8", but I need to know how you figure the math and I am sure someone here is good with fraction math.

Thanks!

 

[Aurora63]

Draw a line that is 2 7/8" long. Mark off each eighth. Then, using a ruler, mark half an inch of the line very dark, maybe black ink. Then have him count the eights that are left, and convert that into a mixed number.

Hope that makes sense!

 

[beckmrk04]

cancel- did my math wrong.. maybe i shoudn't help....

 

[Planogirl]

Convert 1/2" to 8ths which is 4/8". Then subtract from your original number.

 

[DHSsenior007]

Change 1/2 to 4/8 by multiplying the top and bottom by 4, you need a common denominator. Then subtract 7/8 from 4/8. The teacher might need your son to change 2 7/8 to a plain fraction (versus a mixed number), and you would do that by multing 8 by 2, getting 16, then adding that to 7, so the fraction is now 23/8, then you could subtract 7/8 from that. Then, of course, you would have to change it back to a mixed number, by doing the same steps backwards...

Hope this helps!

 

[tiggerlover]

Draw a line that is 2 7/8" long. Mark off each eighth. Then, using a ruler, mark half an inch of the line very dark, maybe black ink. Then have him count the eights that are left, and convert that into a mixed number.

Hope that makes sense!

That is how I got my answer.

 

[tiggerlover]

Thanks to you all.

 

[Lanshark]

I converted 1/2 to 8ths. It is 4/8ths. 2 7/8 - 4/8 = 2 3/8

 

[tiggerlover]

Change 1/2 to 4/8 by multiplying the top and bottom by 4, you need a common denominator. Then subtract 7/8 from 4/8. The teacher might need your son to change 2 7/8 to a plain fraction (versus a mixed number), and you would do that by multing 8 by 2, getting 16, then adding that to 7, so the fraction is now 23/8, then you could subtract 7/8 from that. Then, of course, you would have to change it back to a mixed number, by doing the same steps backwards...

Hope this helps!

But why did you chose to multiply by 4?

Also, I understand how you get 23/8, but when you subtract 7/8 from that you do not get 3/8...what am I missing.

Sorry to be a dunce, but fractions always got the better of me in school.

 

[Planogirl]

2 7/8" long and you erase 1/2"

You have 2 7/8 or 2 + 7/8. This is the same as 16/8 + 7/8 which equals 23/8. Note that the denominator stays the same here.

Then you multiply 1/2 by 4/4 to get the denominator the same as the other number's. Note that 4/4 is the same as 1 so it's like multiplying 1/2 by 1. The answer is 4/8.

Subtract 4/8 from 23/8 and you get 19/8. To simplify think of whole numbers or 8/8. You get 16/8 + 3/8 which is the same as 2 + 3/8 or 2 3/8.

 

[Bayshore Bandit]

Can our country please move to the metric system

Ok, back to your regulary scheduled thread

 

[DVCLiz]

But why did you chose to multiply by 4?

Also, I understand how you get 23/8, but when you subtract 7/8 from that you do not get 3/8...what am I missing.

Sorry to be a dunce, but fractions always got the better of me in school.

You choose to multiply by four because you are trying to make the bottom part of each number the same. Once they are the same, you can perform operations like addition, subtraction, etc. on the top numbers, because you know you are dealing with the same thing (in this case, on both sides of the problem you are delaing with "eighths.") You want to make the smaller number (the 2 that is on the bottom of 1/2) into an 8, and the way you turn a 2 into an 8 is to multiply 2x4. Whatever you do on the bottom you have to do on the top, too. So on the top, 1x4. You end up with 1/2 x 4/4=4/8.

2 7/8 - 1/2 can be changed to 23/8 - 4/8. You made a mistake when you thought it was minus 7/8. If you subtract 23/8 - 4/8, you get 19/8. 8 goes into 19 twice, so you have 2 , then you have 3/8 left over (2 = 16/8), so your answer is 2 wholes, then 3/8 of a whole (or a frction) left over.