Question

after 15 years Mary's age will be 4 times of her present age find her present age

Answer 1

let her present age is x
x+15=4x
15=4x-x
3x=15
x=5
present age is 5 years

Answer 2

x+15=4x
15=4x-x
15=3x
x=15
3
x=5