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?

300 chickens

Explanation:

Let number of chickens = x

Let number of scheduled days that the farmers stock will last = y

Story 1

Number of chicken=x-15

Number of days = y+4

(x-15) (y+4) = 0

4x-15y-60=0

Story 2

Number of chickens = x+20

Number of days = y-3

(x+20) (y-3) = 0

-3x+20y-60=0

Form (1) and (2)

4x-15=-3x+20

7x=35y

X=5y

Sub x = 5y into (1)

20y-15y-300=0

5y=300=x