Description for Correct answer:
Let the required length be x m.

More men, More length (Direct proportion)

Less days, Less length (Direct proportion)

Men 20 : 70

:: 56 : x

Days 6 : 3

Therefore, \( \Large \left(20 \times 6 \times x\right) = \left(70 \times 3 \times 56\right) \)

\( \Large x = \frac{70 \times 3 \times 56}{20 \times 6} = 98\ m \)