A man, a women and a boy can do a job in 3, 4 and 12 days , respectively. How many boys must assist one man and one women to complete the job in one and a half days?

Portion of work completed by one man in 1¹/₂ days.
i.e., 3/2 days = 3/(2 x 3) = 1/2
Portion of work completed by one women in 1¹/₂ days.
i.e., 3/2 days = 3/(2 x 4) = 3/8
∴ Portion of work completed by boys = 1 - (1/2 + 3/8) = 1/8
Now, portion of work completed by each boy in 3/2 days = 3/(2 x 12) = 1/8
Hence, only one boys would be required.