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.

Answer 1

You've already posted the exact same question, to which I replied.

You also posted an earlier, similar question, which was answered. Did you not learn from the process on the other question?

Answer 2

they each do a fraction of the task, based on their individual times ... the fractions sum to one (the whole task)

t/21 + t/28 + t/48 = 1 ... LCD is ... 21 * 16

16t/336 + 12t/336 + 7t/336 = 336/336

35t = 336

Answer 3

noted