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:

$\frac{7}{25}$

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 $\frac{7}{25}$