Whenever 2 quantities (suppose a and b) are inversely proportional to each other, we can write an proportionality equation (with k as proportional constant) as:
Since, here t is INVERSELY PROPORTIONAL to r, we can say:
Firstly, we have t = 2 hour and r = 40 mph, so we find k:
Now, secondly, we have r = 45 and k = 80, we need to find t: