Question

find a number whose one third part exceeds it's one fifth part by 20

Answer 1

let the no be x
1x/3=1x/5+20
x/3-x/5=20
(5x-3x)/15=20
2x=300
x=150
justification 
150/3=50
150/5=30
50=30+20....

Answer 2

Suppose the number required is x, then;
x/3=(x/5)+20
(x/3)-(x/5)=20
(5x-3x)/15=20
2x=20*15
x=300/2
x=150
So, the number required is 150.

Hope this helps you!!