​​The ratio of the present ages of A to B is 7:3. The ratio of A's age 4 years ago to B's age 4 years hence is 2:1. The difference between A's age 4 years hence and B 's age 4 years ago is:​

Correct option is B

​Given:

Present age ratio A : B = 7 : 3

4 years ago to 4 years hence ratio:

​$$\frac{A - 4}{B + 4} = \frac{2}{1}$$​​

Solution:

Let  A = 7x, B = 3x 

Then, 

​$$\frac{7x - 4}{3x + 4} = \frac{2}{1} \\ \ \\ \Rightarrow \quad 7x - 4 = 2(3x + 4)$$​​

​$$7x - 4 = 6x + 8 \\ \ \\ \Rightarrow \quad x = 12$$​​

A's age = 7x = 84 year, 

B's age = 3x = 36 year
Difference = (84 + 4) - (36 - 4) = 56 year​​​​