Layla likes to walk on a treadmill to stay in shape. In addition to her walk, she warms up and cools down with some stretches. Let t be the total length of her workout, w be the length of her warm-up, and c be the length of her cool-down, all in minutes. Then twc is the time she spends on the treadmill. This morning, Layla worked out for 40 minutes, including her 5-minute warm-up and 7-minute cool-down. How many minutes did Layla walk on the treadmill this morning?
A) 28
B) 30
C) 33
D) 35