There is direct proportion and inverse proportion. An example of direct proportion is: if it takes me 30 mins to travel 25 miles, how long will it take me to travel 75 miles? The usual way of solving this is to write 75/25*30 mins= 90 mins. The proportion of miles (in this case three times as many) is used to multiply the time. The greater the distance the longer the time.
Inverse proportion means that you invert the fraction representing the proportion. An example would be: if my average speed is 25mph and it takes me 40 minutes to get to work at that speed, how long will it take me to get to work if my average speed is 50 mph. The proportion is 2 (twice the speed), but at twice the speed it takes half the time, not double. That's inverse proportion. The higher the speed the shorter the travelling time.