Question

A,B and C can do a piece of work in 21 days,42 days and 14 days respectively. A started the work. B joined him after 3 days. If C joined them after 5 days, for how many days did A work?

Answer 1

for first 3 days only A worked , The worked remained after 3 days is 1-(1/21*3=1/7)=6/7.
now B joined and the accuracy became 1/21+1/42=3/42. smilarly a worked with b for 5 days . and with C, A worked for 8 days .
the total days A worked = 22 days

Answer 2

first A work alone for 3 days .then B joined and they work for 5 days and with c they worked for 8 days the total days worked by A is 22 days.