let c=number of chickens

f = stock of feed

p = days that the feed is planned to last

you are told that

(c-15)/f=p+4

(c+20)/f = p-3

c/f = p

thats 3 equations in 3 unknowns, solve simultaneously.

editalarmingly, the equations i wrote have no solutions according to wolfram alpha, so i must have made a mistake