List the multiples of both 12 and 9:
12, 24, 36, 48, 60
9, 18, 27, 36, 45
The lowest common multiple is 36.
This will make the fixtures and bulbs equal.
This means they bought 3 packages of light figures 3 x 12 = 36)
And bought 4 packages of bulbs ( 4 x 9 = 36)