Counting Kangaroos

Can you work out how many kangaroos Rachel draws in one colour?
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Problem



Rachel draws 36 kangaroos using three different colours.

25 of the kangaroos are drawn using some grey, 28 are drawn using some pink and 20 are drawn using some brown.

Five of the kangaroos are drawn using all three colours.

How many kangaroos did she draw that use only one colour?

This problem is taken from the UKMT Mathematical Challenges.