let say t= total marks (addition of all the marks).
here we have given 51 students (n=51)
let say the average is x.
so according to the standard formula of finding the average t/n=x
now, we can use any of the conditions
according to case one:
now take t=51x and solve it
here we get the same answer from the case 2 as well.
Answer : x=60.