Humans and robots can both perform a job but at different efficiencies. Fifteen humansÂ and five robots working together take thirty days to finish the job, whereas five humans and fifteen robots working together take sixty days to finish it. How many days will fifteenÂ humans working together (without any robot) take to finish it?

- 36
- 32
- 45
- 40

### Answer

Let the rates of work of each human and each robot be H and R respectively.

15H + 5R = 1/30

5H + 15R = 1/60

Multiplying first equation by 3,

45H + 15R = 1/10

Subtracting the second equation,

40H = 1/10 - 1/60

40H = 1/12

H = 1/480

In a day, 15 humans can complete 15H i.e. 1/32^{th} of the job.

15 humans can complete the job in 32 days.

**The correct option is B.**