Wow i just cant seem to get any of these word problems right. I just dont have a good formula, and i guess im having a hard time trying to figure out the question itself...although i know the question is not complicated i just cannot find a forumula or an answer!

any help?

Heres the Q's First one:

A chemist has one solution that is 40% alcohol and another that is 55% alcohol. How much of each must she use to make 15 liters of a solution that is 50% alcohol?

Second:

An electronics store put two types of car radios on sale. One model sold for $87, and the other sold for $119. During the sale, the receipts for the 25 radios sold were $2,495. How many of the less expensive radios were sold?

stuff is so confusing to me...but i wanna master it, just need some help

Thanx guys