Question

after 21 years Salman will be 4 times as old as he is now find his present age

Answer 1

21+x=4x
21=4x-x
21=3x
21/3=x
7=x
therefore his present age is 7

Answer 2

Let his orignal age be X
then 21 years later his age will be X +21
at that time he would 4 times his current age
then
X +21 = 4X
3X= 21
X = 7