Shyam can do a job in 20 days, Ram in 30 days and Singhal in 60 days. If Shyam is helped by Ram and Singhal every 3rd day, how many days will it take for them to complete the job?
12
16
15
10
Explanation:
As Shyam is helped by Ram and Singhal every third day, Shyam works for 3 days while Ram and Singhal work for 1 day in every 3 days.
Therefore, the amount of work done in 3 days by Shyam, Ram and Singhal:
=> 3/20 + 1/30 + 1/60
=> 1/5 of the job.
Hence, it will take them 5 times the amount of time = 3 $\times$ 5 = 15 days.