Solve for y: (12, y) and (4, 7)

The answer is 10
I'm going to list the locker numbers by round:
Evens) 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, etc.
2's) 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, etc.
5's) 5, 10, 15, 20, 25, 30, 35, 40, etc.
The first number to repeat is 10, so that would be the first locker Brandy opens.
Hope this helped :)