At Marv's Pet store, there are m mice and f ferrets. If he sells 20 mice, there will still be twice as many mice as ferrets. If he sells 20 ferrets, there will be 14 times as many mice as ferrets. How many ferrets does Marv's Pet Store have?
m-20 = 2f
14(f-20) = m
14(f-20) = 2f+20
14f - 280 = 2f+20
12f = 300
f = 25