A and B can do a work in 15 days and 20 days respectively. If they work together for 4 days, then the fraction of the work that is left is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(\frac{8}{15}\)
A person who finishes a job in \(n\) days does \(\frac{1}{n}\) of it in one day, and when two work together their one-day work is added.
One-day work of A and B together \(\displaystyle =\frac{1}{15}+\frac{1}{20}=\frac{4+3}{60}=\frac{7}{60}\)
Work done in 4 days \(\displaystyle =4\times\frac{7}{60}=\frac{7}{15}\)
Work left \(\displaystyle =1-\frac{7}{15}=\frac{8}{15}\)
Option (3), \(\frac{7}{15}\), is the part of the work done in the 4 days, not the part left.