The ratio for the cyclist's speed is 12 km/1 hr. If I were to find the ratio for kilometres per 1.5 hour, I would need to multiply the denominator of 1 (aka 1 hour) by 1.5 to get (1.5 hours). Whatever you do to the bottom, you have to do to the top. So I multiply the numerator of 12 (12 km) by 1.5 to get 18 km.
