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A can do a piece of work in 10 days and B in 15 days. Working together they finish it in:
A5 days
B7.5 days
C6 days
D12.5 days
Answer & Solution
Correct answer: C. 6 days
1. Work with the fraction each finishes in one day.
2. A does one tenth of the work per day and B does one fifteenth.
3. Together they do 1/10 + 1/15 = 3/30 + 2/30 = 5/30, which is one sixth per day.
4. Completing the whole job therefore takes 6 days.
5. Averaging 10 and 15 to get 12.5 is the classic error; two people together must be faster than either alone.
_Source: NCERT Class 8 Maths, Ch 11 'Direct and Inverse Proportions', S11.3 Inverse Proportion_
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