R is 80% more efficient than S. If S alone can make a book in 90 days, then R alone can make the book in how many days?
Let total work to be done = 900 units
S alone can make a book in 90 days, => S's efficiency = $$\frac{900}{90}=10$$ units/day
=> R's efficiency = $$10+(\frac{80}{100}\times10)=18$$ units/day
$$\therefore$$ Time taken by R alone to make the book = $$\frac{900}{18}=50$$ days
=> Ans - (C)
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