if there is an inverse relationship,
then their product is a constant.
As distance is constant here since he travels the same journey in both instances
if one is 30, the other is 45,
hence if he takes 30 minutes, the speed must be 45 km/hr
Alternatively, an inverse relationship may be written
Yet another way is...
He travels for 45 minutes at 30km/hr
45 minutes is 3/4 of an hour
The distance travelled is km
In 30 minutes, which is half an hour, he travels that distance.
He would have travelled twice that distance in 1 hour.
That's a speed of km/hr, hence his speed is 45 km/hr