the present ages of rohan and sohan are in ratio 4:5.four years hence their ages will be in ratio 5:6.find their present ages

let the Rohan's present age = 4x yrs

so, Sohan's present age = 5x yrs

After 4 years :

Rohan's  age = ( 4x + 4 ) yrs

Sohan's age = ( 5x + 4 ) yrs

According to the question, 

$\frac{4x + 4}{5x + 4}= \frac{5}{6}$
$\Rightarrow$6(4x + 4) = 5(5x + 4)
$\Rightarrow$24x + 24 = 25x + 20
$\Rightarrow$25x - 24x = 24 - 20
$\Rightarrow$x = 4
so, Rohan's present age = 4x = 4(4) = 16 yrs
Sohan's present age = 5x = 5(4) = 20 yrs