If a man walks 20 km at 5 km/hr. he will be late by 40minutes. If he walks at 8 km/hr, how early from the fixed time will he reach?

The distance traveled with speed of 5 km per hour is 20 km, so the time taken to travel this is $\frac{20}{5}=4 \mathrm{hours}$

Hence, the time taken to travel same 20 km in 8 km per hour speed is $\frac{20}{8}=2.5 hours$

4 hours time is 40 min late, then 2.5 hours is $1.5 \mathrm{hours} - 40 \min =90-40=50 \min$