20 labourers working 7 hours a day can finish a piece of work in 30 days If the labourers work 5 hou...........

Quant (SSC) Time and work

20 labourers, working 7 hours a day can finish a piece of work in 30 days. If the labourers work 5 hours a day, then the number of labourers to finish the same piece of work in 40 days, will be :

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Explanation:

Let the required number of labourers be z. then,
Less working hrs/ Day, More labourers (Indirect proportion)
More days, Less labourers (Indirect proportion)
Hours 5 : 7
:: 20 : z
Days 40 : 30
z = 7 x30 x 20 / 5 x 20 = 21 labourers.