D alone can do a work in 20 days and E can do the same work in 25 days. If they work on it together for 8 days, then what fraction of the work is left:
Answer & explanation
Correct answer: option 2
efficiency ∝ $\frac{1}{time\;taken}$
D : E
Time taken 20 : 25
Efficiency 5 : 4
Total efficiency = 4 + 5 = 9
Total work = LCM of 20, 25 = 100
Work completed by C and D in 8 days = efficiency x number of days
= 9 x 8 = 72
Work remaining = 100 - 72 = 28
This is same as $\frac{28}{100}$ or $\frac{7}{25}$