Well, let's suppose the average was over N subjects, then the total marks over these subjects would be 90N, because if we divide by N we get the average of 90. Now we add 60=90N+60. This time we have N+1 subjects so the average is (90N+60)/(N+1). We don't know N so we can't find the average. Let's see an example. If N=5 then 90N=450 and the total score from 6 subjects is 450+60=510 and the average is 510/6=85.
So you need to supply N to solve the problem.