A man covers a distance in 6 hours at 40 km per hour. To cover it in 4 hours he must travel at:

1. First find the distance: 40 x 6 = 240 km. 2. The distance does not change, so speed and time are in inverse proportion. 3. The required speed is 240 / 4. 4. That is 60 km per hour. 5. Checking the ratio, the time fell to two thirds, so the speed must rise to three halves: 40 x 3 / 2 = 60. _Source: NCERT Class 8 Maths, Ch 11 'Direct and Inverse Proportions', S11.3 Inverse Proportion_