fair 6-sided dice are simultaneously rolled. The probability that exactly two of them show a number 1 is 25/72. Find n.

I think your probability should read: 5 / 72 NOT 25 / 72. If that is correct, then:

[(nC2) * 5^(n - 2)] / 6^n =5 / 72, solve for n

n = 3 dice.