Cleaning a car.
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The way to calculate this is to figure out what fraction each does in an hour (1/20 of the job, vs. 1/25 of the job). Add the fractions together to see how much they do together in an hour, then take the reciprocal to find out how many hours it takes them.
It will take about 11.1 hours for the two of them to complete the roofing of the house.
Well, darling, if one person takes 3 hours and the other takes 6 hours to paint the fence, then when they join forces it will take them 2 hours to finish the job together. It's simple math, honey. Just add their individual rates of work to get the combined rate and then divide the total job by that rate. Easy peasy lemon squeezy!
9 men taking 5 hours = 9 x 5 = 45 hour job 3 men doing a 45 hour job = 45 / 3 = 15 hours
It should take 15 electricians provided that two unlikely conditions are satisfied:all electricians work at exactly the same rate, andthe job contains no parts which are time-dependent. For example, if an instalment must be tested for 5 hours, then even infinitely many electricians will not do the job in 4 hours.