There were 57 cars in a parking lot then 34 more cars drove in and part some cars drove away now there are 66 cars in the parking lot how many cars drove away

Serena is making an experiment. for that, she needs 20 grams of a 52% solution of salt. she has two large bottles of salt water: one with 40% and the other with 70% of salt in them. how much of each must she use to make the solution she needs? i am in need of