Helpo me please

Does anyone know how to do geometryme with this problem

Anut store normally sells cashews for? $4.00 per pound and peanuts for? $1.50 per pound. but at the end of the month the peanuts had not sold? well, so, in order to sell 40 pounds of? peanuts, the manager decided to mix the 40 pounds of peanuts with some cashews and sell the mixture for $2.00 per pound. how many pounds of cashews should be mixed with the peanuts to ensure no change in the? profit?

You are given the first four terms of an arithmetic sequence. which one is best to use, recursive formula or explicit formula? why? pleasseee ! : (