OE


We know that 4 persons take ' $h$ ' hours each to complete a work, so their speed is $\(\frac{1}{h}\)$ each and 3 persons take ' $2 h$ ' hours each to complete the same amount of work, so their speed is $\(\frac{1}{2 h}\)$ each. $(\because$ Work done $=$ Speed $\(\times$ Time $)\)$

Now, the combined speed of all 7 persons is $\(\frac{4}{h}+\frac{3}{2 h}=\frac{11}{2 h}\)$ which means the time taken by all 7 person is $\(\frac{2 h}{11}\)$ (Time \& speed are inversely proportional when work is constant)

We know that the time taken by all 7 persons to complete the work is 140 hours, so we get $\(\frac{2 h}{11}=140 \Rightarrow \mathrm{~h}=70 \times 11=770\)$ hours

Finally the mean of the time taken by the 7 persons is $\(\frac{ Total time }{ Number of persons} =\frac{4 h+3(2 h)}{7}\)$ $\(=\frac{10 h}{7}=\frac{10 \times 770}{7}=1100 \&\)$ the median time is $\(\mathrm{h}=770\)$ hours.

Hence the difference between the mean \& the median time is $\(1100-770=330\)$, so the answer is (D).