Question 563486
Let the number of sheep be x
Let the number of chikens be x

x+y = 21.......................(1)

we all know sheep has 4 legs and chicken has two
the number of legs  are56

4x+2y=56.......................(2)

Multiply equation (1) by -2
we get -2x-2y=-42

add this equation to (2)
-2x-2y=-42
4x+2y=56
2x= 14
/2
x=7 , number of sheep
so the number of chickens will 21-7 = 14

CHECK
14*2+7*4=56

cheers

m.ananth@hotmail.ca