alleryA and B together finish a job in 12 days. A alone takes 20 days. B alone would take:
A24 days
B28 days
C30 days
D32 days
Answer & Solution
Correct answer: C. 30 days
1. Together they complete one twelfth of the work each day.
2. A alone completes one twentieth each day.
3. So B's daily share is 1/12 - 1/20.
4. Taking 60 as the common denominator, that is 5/60 - 3/60 = 2/60, which is one thirtieth.
5. B alone therefore needs 30 days, which is sensibly slower than A.
_Source: syllabus Maths, Ch 11 'Direct and Inverse Proportions', S11.3 Inverse Proportion_
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