(riddle maths 10 points)..?

Asad1

Member
shehryar can do a job in 20 days,rahim in 30 days and saeed in 60 days.. If sheharyar is helped by rahim and saeed every 3rd day. how long will it take for them to complete the job?

ans 15
i want to know "how".....?
 
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