B2 This month, I spent 26 days exercising for 20 minutes or more, 24 days exercising 40 minutes or more, and 4 days of exercising 2 hours exactly. I never exercise for less than 20 minutes or for more than 2 hours.
What is the minimum number of hours I could have exercised this month?
Answer: 22
Solution: From the four days where I exercised 2 hours, I exercised a total of 8 hours. There are $24-4=20$ other days where I exercised at least 40 minutes; this contributes a minimum of 800 minutes, or 13 hours and 20 minutes. Finally, there are $26-24=2$ more days where I exercised at least 20 minutes; this contributes a minimum of 40 minutes, for a total of $8+13+1=22$ hours.

Alternative representations of the verbal solution:

Calculation in minutes:
$$
\frac{[(26-24) \times 20+(24-4) \times 40+4 \times 120] \text { minutes }}{60 \mathrm{~min} / \mathrm{hr}}=\frac{840+480}{60}=22 \text { hours. }
$$

Calculation in hours:
$$
(26-24) \times \frac{1}{3}+(24-4) \times \frac{2}{3}+4 \times 2=14+8=22 \text { hours. }
$$

Graphical representation of minimum hours of exercise:
Reading off the graph of exercise horizontally, we have (in hours) $26 \times \frac{1}{3}+24 \times \frac{1}{3}+4 \times \frac{4}{3}$ $=\frac{66}{3}=22$ hours.