1 Answers
Option 3 : 7 days
Given:
A can complete the work in 14 days
B can complete the work in 20 days
A + B together work for some days and then A left
Total work completed in 10 days
Calculation:
A = 14 days
B = 20 days
Total work = LCM(14,20) = 140 unit
⇒ One day's work of A = 140/14 = 10
⇒ One day's work of B = 140/20 = 7
Let A and B worked together for X days, and then A left
⇒ B alone worked for (10 - X) days
Total work = ((10 + 7) × X) + (7 × (10 - X)) = 140
(17 × X) + 70 - (7 × X) = 140
(17 × X) - (7 × X) = 140 - 70
10 × X = 70
X = 7
⇒ A and B worked together for 7 days and then A left
∴ A left after 7 days
Shortcut Trick
LCM of A and B is 140
The efficiency of A = 10
The efficiency of B = 7
B worked for 10 days
= 7 × 10 = 70
Remaining work done by A
= (140 - 70)/10 = 7 days.
∴ A left after 7 days