A work can be completed by P and Q in 12 days, Q and R in 15 days, R and P in 20 days. In how many days P alone can finish the work?
P+Q will do $$\frac{1}{12}$$ amount of work in a day.
Q+R will do $$\frac{1}{15}$$ amount of work in a day.
R+P will do $$\frac{1}{20}$$ amount of work in a day.
So P+Q+R will do $$\frac{1}{10}$$ amount of work in a day.
or P+ $$\frac{1}{15}$$ = $$\frac{1}{10}$$
So P will do $$\frac{1}{30}$$ amount of in a day.
Hence complete work will be done in 30 days.
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