If the time it takes to cover a distance by car varies inversely with the speed of the car, then we can use the formula:
time = k / speed
where k is a constant of proportionality. We can find the value of k by using the information given in the problem.
When the car is moving at 35 mph, the time it takes to complete the trip is 9 hours. So we can write:
9 = k / 35
Multiplying both sides by 35, we get:
k = 315
Now we can use this value of k to find the time it takes for a car moving at 21 mph:
time = 315 / 21
time = 15 hours
Therefore, it would take a car moving at 21 mph a total of 15 hours to cover the same distance that a car moving at 35 mph covers in 9 hours.