Answer:
They would get a haircut on the same day in 72 days.
Explanation:
Use least common multiple
3 l 9 / 24 / 12
4 l 3 / 8 / 4
____________
3 / 2 / 1
LCM: 3*4*3*2*1 = 72
8.8m questions
11.4m answers