1 Answers
Option 3 : 4xy/3(y + x)
Adam < Eve < Kane in terms of efficiency.
Adam and Eve together take x days.
Kane and Eve together take y days.
One of them, if he worked alone would take thrice as much time as it would take when all
three work together. This is a crucial statement. Now, if there are three people who are all
equally efficient, for each of them it would take thrice as much time as for all three
together.
Now, this tells us that the person who takes thrice as much time cannot be the quickest one.
If the quickest one is only one-third as efficient as the entire team, the other two cannot
add up to two-thirds. By a similar logic, the slowest one cannot be the person who is one-
third as efficient.
In other words, the person one-third as efficient = Eve
Let Adam, Eve, and Kane together take m days. Eve alone would take 3m days
Adam and Eve together take x days. Or Adam + Eve in 1 day do 1/x of the task ----(1)
Eve and Kane together take y days. Or, Adam + Eve in 1 day do 1/y of the task ----(2)
Eve takes 3m days to do the task. Or, Eve, in one day, does 1/3m of the task ----(3)
Now, if we do (1) + (2) – (3) we get
Adam + Eve + Kane do 1/x + 1/y – 1/3 m in a day. This should be equal to 1/m as all three of them complete the task in m days.
1/x + 1/y – 1/3m = 1/m
⇒ 1/x + 1/y = 4/3m
⇒ (x + y)xy = 4/3m
⇒ 4xy/3(x + y) = m
⇒ m = 4xy/3(y + x)
∴ Time taken by all three if they work together = 4xy/3(y + x)