P, Q, and R can complete a certain work in 24, 36, and 72 days, respectively. How many days will it take for P to finish the work if he receives help from Q and R every fourth day?

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- **Rates:** P = $1/24$, Q = $1/36$, R = $1/72$. So Q+R = $1/24$ and P+Q+R = $1/12$.

- **Work in 4 days:** If a help day occurs then three P-alone days, work = $1/12 + 3	imes(1/24)=5/24$.

- **After 16 days (4 blocks):** Work done = $4	imes(5/24)=20/24$. Remaining = $1-20/24=4/24$.

- **Finish remaining:** Day 17 is a help day (gives $2/24$), day 18 P alone gives $1/24$ (remain $1/24$), day 19 P alone finishes. Total = **19 days**.

FINAL_ANSWER: d

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