Question

An athlete can burn 10 calories per minute while running and 4 calories per minute while walking. If the athlete burns 450 calories in 60 minutes, how long is spent on each activity?

Answer 1

w = minutes running
r = minutes running

You can get two equations from the information given:

10r + 4w = 450; // total calories burned = 450
r + w = 60 // total minutes = 60

it follows that:
r = 60 - w

10r + 4w = 450
10(60 - w) + 4w = 450
600 - 10w + 4w = 450
600 - 6w = 450
-6w = -150
w = 25

plug into r + w = 60 to get r
r + 25 = 60
r = 35

so:
w = 25
r = 35