What they called chicken that crossed the road

Jerrold has to buy 12 copies of a textbook which costs $36.25 per copy. how much must he spend for them?

If you are in grade when should you start studying for exams

Raul has one solution that is 20% acid by volume and one solution that is 50% acid by volume. he plans to mix together some of each solution to create 30 milliliters of a solution that is 40% acid by volume. how many milliliters of the 20% acid solution should raul use to create the 40% acid solution? can someone explain this to me? i know that the answer is 10 milliliters, but i don't know how to get it.