We will solve this using the LCM technique.
A takes 14 Days & B takes 21 Days. So, let the total work be LCM(14,21) = 42 Units
A -> 14 days = 3 Units/day
B -> 21 days = 2 Units/day
A + B = 5 Units/day
Now, for the last 3 days , only B worked as A left.
So, work done in the last three days = 2×3= 6 units
Work done by A & B together = 42–6 = 36 units
Time taken = 36/5 = 7.2 days
Thus, TOTAL TIME = 3+7.2 = 10.2 Days