In the year 2000, Monu was 3 times his sister's age. In 2010, he was 24 years older than her. Find Monu's age in 2010.

Correct option is B

Given:
In the year 2000, Monu's age was 3 times his sister's age.
In 2010, Monu was 24 years older than his sister.

Solution:

Let M be Monu's age in 2000.

Let S be his sister's age in 2000

From the first statement,

​$$M=3S$$​​

From the second statement,

​$$(M + 10) = (S + 10) + 24$$​​

Substituting M = 3S from the first equation into the second equation:

​$$(3S + 10) = (S + 10) + 24$$​​

​$$2S=24$$

$$S=12$$

 In 2000, Monu's sister was 12 years old.

So, Monu =  3 $$\times$$​ 12 = 36 years 

Now, 

Monu's age in 2010 = 36 + 10 = 46 years old.