# Thread: Ratio/Proportion: Given: A/B=7/9 and C/D=5/3; If B=4D, compute A+B/C+D

1. ## Ratio/Proportion: Given: A/B=7/9 and C/D=5/3; If B=4D, compute A+B/C+D

The problem is "Given: A/B=7/9 and C/D=5/3; If B=4D, compute A+B/C+D". It's not homework, just a practice problem, so no rush (not that there would be anyway, but you know). I don't really understand how to start. I've tried a lot of things, basically just creating equations and substituted, but nothing has worked. I'd appreciate if someone could show me how to begin finding the variables. Thanks!

2. Originally Posted by lei
… Given: A/B=7/9 and C/D=5/3; If B=4D, compute (A+B)/(C+D)

… I've tried a lot of things, basically just creating equations and substituted, but nothing has worked.
Those grouping symbols are very important; please ask, if you don't understand why.

You were probably on the right track, with some of your starts; maybe you didn't go far enough, before starting over. You didn't show any work, so I'm not sure what you tried.

Setting the cross-products equal to one another might be a good start.

9A = 7B

3C = 5D

Replace that B with 4D, and simplify: 9A = ?D

Expressions 9A and ?D represent the same number. We can divide each side of the equation 3C=5D by that number.

There are other ways to begin; I showed one example.

3. ## Thanks!

Originally Posted by mmm4444bot
Those grouping symbols are very important; please ask, if you don't understand why.

You were probably on the right track, with some of your starts; maybe you didn't go far enough, before starting over. You didn't show any work, so I'm not sure what you tried.

Setting the cross-products equal to one another might be a good start.

9A = 7B

3C = 5D

Replace that B with 4D, and simplify: 9A = ?D

Expressions 9A and ?D represent the same number. We can divide each side of the equation 3C=5D by that number.

There are other ways to begin; I showed one example.