Now that we have found the age of the father, which is 80 years, we can substitute this value back into the equation to find the age of the son.

Suppose present ages of Father = F, and Son = S

• Means, F + S = 130
• Or S = 130 – F
• n years back, Father’s age: F – n, and Son’s age: 130 – F – n

Given, father said to his son, “n years back I was as old as you are now.”

• Means, F – n = S
• Or F – n = 130 – F
• 2F = 130 + n
• Or n = 2F – 130 …… (i)

Given, father said to his son, "My present age is four times your age n years back".

• Means F = 4 (130 – F – n)
• Substituting n from equation 1: F = 4 (130 – F – 2F + 130)
• Or, F = 4 (260 – 3F)
• Or, 13F = 4 * 260
• Or F = 80 years
• Hence, Son's age = S = 130 – F = 50 years

Difference in ages: 

• = 80 – 50 = 30 years

Therefore, the correct answer is (a) 30 years.