R can do a piece of work in 20 days and K in 25 days. They work together for 5 days and then K leaves. In how many days would R finish the work?
The Usual Method
R can finish of the work in 1 day.
K can finish of the work in 1 day.
Thus total work finished by R and K working together for 5 days = .
Hence remaining work = .
Since R has to work alone now and R can finish of the work in 1 day, so for R to finish of the work, it will take,
By cross multiplication, we get:
11 days more.
Estimated Time to arrive at the answer = 45 seconds.
R will finish or 25% of the work in 5 days. K will finish or 20% of the work in 5 days. So, work remaining to be done = 100% – (25% + 20%) = 55%.
Since R takes 5 days to finish 25% of the work, he will take 10 days to finish 50% of the work. So for 55%, R will take a little over 10 days, so 11 days to finish. (Also, 55% is a multiple of 11, so the answer should also be a multiple of 11.)
Estimated Time to arrive at the answer = 10 seconds.