Timios

Mixture #1
3 cups of water mixed with 5 cups of milk in one container.

Mixture #2
7 cups of water mixed with 8 cups of milk in another.

Mixture #1 is to be mixed with Mixture #2 in the ratio of 4:7

So 4 times Mixture #1 would be 12:20 (12 cups of water and 20 cups of milk)
And 7 times Mixture #2 would be 49:56 (49 cups of water and 56 cups of milk)

Now add all the water together, and add all the milk together, and from that you have the required ratio.

skipjack

That's incorrect and doesn't give the ratio. However, the ratio wasn't required, just the reason why 4:7 wouldn't be correct, which has already been explained.