UPSC 2014 CDS I Math Paper

Let the present age of Ravi be x years.
So, present age of Ravi’s father= 4x years
Five years ago, Ravi’s father’s age = 7 × Ravi’s age
∴4x−5=7(x−5)
⇒7x−4x=35−5
⇒3x=30
⇒x=10
Therefore, present age of Ravi’s father =4×10=40 years