The table shows the distribution of the time (in minutes) spent by students on daily physical exercise. One class frequency is missing.
Time (min)
10–20 | 20–30 | 30–40 | 40–50 | 50–60
Frequency
5 | 12 | k | 11 | 7
Total number of students = 45.
a. (i) Determine the missing frequency k.
b. (ii) Using the completed table, calculate the standard deviation of the time spent (correct to two decimal places).