What is the difference between direct proportion and indirect proportion?

Answer 1

Two variables are said to be in direct proportion if, when you increase one by some multiple, the other also increases by that same multiple. Alternatively, both decrease by the same proportion.

Two variables are in indirect (or inverse) proportion if an increase in one of them is associated with a decrease in the other, and conversely. The proportional change in both is the same.

For example, if I change the speed at which I am driving, then the distance that I can cover in a fixed amount of time will change in the same proportion. Double my speed then double the distance, quadruple speed and quadruple distance, halve the speed and halve the distance. So speed and distance are directly proportional.

Now think of speed and the time taken to do a fixed distance. Double my speed and halve the time. Halve the speed then double the time. So time taken is inversely proportional to speed.