arithmetic - antony can do a certain job in 21 days david is 40 more efficient than antony how many days does david alone take to do the same job - skillgun

Solution: Ratio of times taken by Antony and David = 140 : 100 = 7 : 5.
suppose David alone takes x days to do the job.
Then, 7 : 5 : : 21 : x
=> 7x = 21*5
=> x = 15 days.