One formula, three ways round
speed = distance ÷ time
distance = speed × time
time = distance ÷ speed
Keep the units matched: km and hours give km/h. 40 minutes is 40/60 = 2/3 of an hour, not 0.4 hours.
Changing units, and average speed
km/h to m/s: × 1000 (metres) then ÷ 3600 (seconds). 72 km/h = 72,000 m in 3600 s = 20 m/s.
Average speed for a whole journey is total distance ÷ total time, never the mean of the speeds. 60 km at 30 km/h then 60 km at 60 km/h: 120 km in 3 hours, so 40 km/h, not 45.
Check a conversion
Use the unit converter to change 72 km/h into m/s, then 15 m/s into km/h. Check each against × 1000 ÷ 3600.