This is an aptitude based question.Let us assume that x is the age of A and y be the age of B.So four years later their age will become  x+4 and y+4.It is given that the ratio of x+4 and y+4 is  8:9.By this we get to know that the age of B is more than age of A. Let’s  take m as the common variable for the ratio.We need to take m because 8:9 might be formed by dividing into lowest term.But one thing that can be determined is that after four years age of A will be in multiple of 8 and age of B will be in multiple of 9

x+4=8m and y+4=9m

Subtracting 4 on both sides for both the equations

we get x=8m-4 and y=9m-4.—–Eq.1

Now we have been given that average of age of A and B is 47 years.So

(x+y)/2=47

Multiplying by 2 on both sides we can get

x+y=94——-Eq.2

Now we can substitute the values of x and y obtained in equation 1 into equation 2.

So. 8m-4+9m-4=94

Which gives 17m-8=94

On adding 8 on both sides we get 17m=102

So we get the value of m as 6 since 102/17=6.

Substituting m in equation we can get value of x and y

x=8m-4=8×6-4=48-4=44

y=9m-4=9×6-4=54-4=50

So we get the present age of A as 44 and B as 50.So the difference in their age is 50-44=6.Hence B is 6 years elder than A