To solve this problem, let's represent the present age of the son as x.
According to the problem, the father is 30 years older than his son, so the father's present age can be represented as (x + 30).
In 5 years, the son's age will be (x + 5), and the father's age will be (x + 30 + 5) = (x + 35).
According to the problem, the father will be thrice as old as the son after 5 years. Mathematically, this can be represented as:
(x + 35) = 3(x + 5)
Now, let's solve this equation to find the value of x:
x + 35 = 3x + 15
Subtracting x from both sides:
35 = 2x + 15
Subtracting 15 from both sides:
20 = 2x
Dividing both sides by 2:
x = 10
So, the son's present age is 10.
To find the father's present age, we can substitute this value back into the expression (x + 30):
Father's present age = 10 + 30 = 40
Therefore, the correct answer is B) 40.