The correct answer is (B):
Let the rates of work of each human and each robot be H and R respectively (both in units/day).
15H+5R = 1/30 ...(1)
5H+15R = 1/60 ....(2)
3(1)-(2) ⇒40H = 1/12
H= 1/480
In a day, 15 humans can complete 15H i.e. 1/32th of the job.
15 humans can complete the job in 32 days.