x/5=x-64

For starters, multiply both sides by 5   

How can i explain for a kid ?

J=5B (1)

J-64=B+64   (2a)

add 64 to both sides

J=B+128       (2b)

(1)=(2b)

5B=B+128

4B=128

b=128/4

b=$32

J = 5*32 = $160