If a farmer sells 15 of his chickens, his stock of feed will last for 4 more days than planned, but if he buys 20 more chickens, he will run out of feed 3 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?
My approach is as follow
the load be defined as the number of chicken x the number of day
let x be the number of chicken and D the number of day
case 1 (X-15) x (D+4)
case 2 (X+20)x (D+3)
from there I am stuck
please help !