A linear equation or a function that varies directly is represented by the following expression:

To determine the constant rate of change between the variables, let's substitute y=315 calories, x=18 minutes

Therefore, an equation that represents the situation would be
Let c be the number of calories he burns
Let m be the number of minutes he uses the treadmill

To determine how many calories would he burn in 25 minutes, we must substitute m=25

After 25 minutes he would burn 437.5 calories