how many athletes exercised more than 40 but less than 60 minutes

The correct answer is:
4.
Explanation:
The number 40 is written in this stem-and-leaf plot as 4|0. 60 is written as 6|0. There are 4 leaves between these numbers; 5, 5, 5 and 5, with the stem of 4. This means the numbers are 45, 45, 45 and 45. There are only 4 between these two numbers.