Kayla has 80 coins in her bank. They are all dimes and nickels. She ha four times as many dimes as nickels. How much money does she have in her bank?

x+4x=80

5x=80

x=16

so she has 16 nickels (and so 64 dimes)

therefore she has 64*0.1+16*0.05 i.e. $7.20