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1. 30 labourers working 7 hours a day can finish a piece of work in 18 days. If the labourers work 6 hours a day, then the number of labourers to finish the same piece of work in 30 days, will be ?

A. 15

B. 21

C. 22

D. 25

Solution:

Let the required number of labourers be x
Less working hour/day, More labours (Indirect proportion)
More days, Less labourers (Indirect proportion)

\begin{matrix} Working hours/day  6 : 7 \\ Days 30 : 18 \end{matrix}} :: 30 : x

\begin{aligned} ∴ 6 \times 30 \times x = 7 \times 18 \times 30 \\ \Leftrightarrow 6 x = 126 \\ \Leftrightarrow x = 21 \end{aligned}

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