Math
posted by Gary .
A man can do a certain task in 21 hours, another man can do the task in 28 hours and a boy can do the task in 48 hours. Find how long it will take to do the task if all three work.

Try figuring out how much of the task each man can do in one hour, as a fraction.
You can use that to find out the work done in an hour if all three work together, and thus the amount of time it will take. 
Man 1/21 other man 1/28 boy 1/48
Man’s rate other man’s rate boy’s rate
1/21 1/28 1/48
Let x be the number of hours it will take all three
to do the task.
Combined rate = 1/x
1/21 + 1/28 + 1/48 =
LCD = 28, 224
1344 / 28, 224 + 1008 / 28, 224 + 588 / 28, 224
= 2 940 / 28, 224 or 735 / 7056
1/x = 735/7056
Cross multiply
735x = 7056
x = 7056/735
x = 9.6 or 9 hours and 10 minutes
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