Answer: 90
Explanation:
Let us say that x student tickets were sold, that would mean x - 25 adult tickets were sold from our second given.
Using our first and third given, we will get the equation
6(x - 25) + 3(x) = 660
6x - 150 + 3x = 660
9x - 150 = 660
9x = 810
x = 810 ÷ 9 = 90
So 90 student tickets were sold