posted by melvin .
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.
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?
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