P, Q and R can do a work in 10, 15 and 30 days, respectively. In how many days can P do the work, if Q and R helped him every third day?

P's 1-day work is 1/10, while the combined 1-day work of P, Q, and R is 1/10 + 1/15 + 1/30 = 6/30 or 1/5. In a 3-day cycle, P works alone for 2 days and with the team on the third day, completing 2/10 + 1/5 = 2/5 of the job. After 2 such cycles (6 days), 4/5 of the job is done, leaving 1/5 of the job, which P completes with Q and R on the 7th day, so P finishes the work on the 8th day working alone.

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