Explanation:

1 labour's 1 days work = 1/96
12 labours 3 days work = (1/8) * 3 = 3/8.
Remaining work 1 - (3/8) = 5/8 ,
15 labours 1 days work = 15/96
Now, 15/96 work is done by them in 1 day
5/8 work is done by them in (96/15) * (5/8) = 4 days.