So is this the right answer?
Yes. Not too hard to confirm with a tiny little program:
lockers = ['Closed'] * 101
for i in range(1, 101):
for locker in range(i, 101, i):
lockers[locker] = 'Open' if lockers[locker] == 'Closed' else 'Closed'
print ([locker for locker, value in enumerate(lockers) if value == 'Open'])
>>> [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
25: 1 5 25 (3)
36: 1 2 3 4 6 9 12 18 36 (9)