Explanation:
1 km = 1000 m = 0.621371 miles
so,
786.3 m = 0.7863 km (simply divide by 1000).
40 km/h = 40×0.621371 mph (as the reference of 1 hour stays the same).
= 24.8548 mph
always consider the direction of the conversion rate : are you going from a smaller to a larger unit, or from a larger to a smaller unit ?
if we would have said
1 m = 0.001 km
then we would have converted 786.3 m by multiplying it by 0.001 (also getting 0.7863 km as result, of course).
but because we defined it the other way around, we had to divide by 1000. it is clear that this is the same operation, yes ?
in the same way, if we would have said
1 mile = 1.60934 km
then we would have converted 40 km to miles by dividing 40 by 1.60934. again getting the same result, of course.