The ratio, or quotient, of direct proportionality is always constant

where k is constant. This means that as one grows, the other will grow at the same proportion.

Example 1

Liisa-Petter had 280 sea cucumbers. They eat 3.5 kg of phytoplankton daily. Liisa-Petter wanted to buy 140 more sea cucumbers for her farm. How much phytoplankton does she need to reserve for all the sea cucumbers per day?


There is a direct correlation between the amount of sea creatures and the amount of food consumption. So we can say that they are directly proportional.

Let's find the proportion and solve x

Answer: About 5.3 kg of phytoplankton should be reserved per day.

The product of an inverse proportionality is also always constant.

where k is constant. This means that as one grows, the other will shrink in the same proportion.

Example 2

Liisa-Petter noticed that the balance in her bank account was inversely proportional to the hours spent at the pub. If she sat in the pub for 5 hours then there was only €200 left in her account. She wanted to sit for an additional 3 hours because the music was so good and the drinks were, of course, cheap. How much money is in the account when Liisa-Petter leaves home?


Let's make a proportion table

Let form a proportion so that the relation is inverted. That is, the ratio of x to 200 is equal to the ratio of 5 to 8.

There is 125 left in the bank account.

Turn on the subtitles if needed