The time taken by a vehicle is 3 hours at a speed of 60 miles/hour. What would be the speed taken to cover the same distance in 4 hours?

Consider speed as m and time parameter as n.

If the time taken increases, then the speed decreases. This is an inversely proportional relation, hence m ∝ $\frac{1}{\mathrm{n}}$.
Using the inverse proportion formula,
m = $\frac{\mathrm{k}}{\mathrm{n}}$
m × n = k
At speed of 60 miles/hour, time = 3 hours, from this we get,
k = 60 × 3 = 180
Now, we need to find speed when time, n = 4.
m × n = k
m × 4 = 180
∴ m = $\frac{180}{4}$ = 45

Therefore, the speed at 4 hours is 45 miles/hour.