A can finish a work in 36 months, B in 18 months and C in 24 months. B and C start the work but are forced to leave after 4 months. The remaining work was done by A in:
22 months
(B + C)`s 1 month`s work = 118+124=772
work done by B and C in 4 months = 772×4=718
Remaining work = 1−718=1118
Now, 1/36 work is done by A in 1 month
So, 11/18 work is done by A in 36×1118=22 months