Question 515474
If 5 bricklayers do the work in 8 hours, then we can model it as each bricklayer does 1/5 of the whole job in 8 hr or 1/40 of the job per hr.
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That means it would take each bricklayer 40 hr to do the job working alone.
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(1/40)*5 = 5/40 = 1/8 of the job per hr, so the whole job takes 8 hr.
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How long will it take 4 bricklayers to build the wall?
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(1/40)*4 = 4/40 = 1/10 per hr.  So it will take 4 bricklayers 10 hr.
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Done.