Two people who share a cellphone plan with 20 gb used all 20 gb last month. if person A used 2 fewer gb than person B, how many gb did person a use

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