framing of equation

Rishu is as much younger to Nanu as he is older to Bhanu. The sum of ages of Nanu and Bhanu is 8.
How old is Rishu now?

Answer:
N- R = R - B 
where, N = nanu's age
R = rishu's age
B = Bhanu's age

Therefore, 2R = N + B 
or, 2R = 8, given N + B = 8
or, R = 4 years.

So, Rishu's age is 4 years.