If you start off multiplying the mean by 3, you'll get the sum of the numbers = 7.5

This means all three numbers add up to 7.5.

So if you take the other two numbers away from 7.5 you'll have the third number.

7.5 - 1.75 = 5.75

5.75 - 1.3 = 4.45.

The third number is 4.45