What is the average commute time for HOV Lane?

Horatio Oscar Vineeth Lane (HOV Lane for short) records his commute times for a period of one month and assigned them to five different categories as shown in the table.

Commute Length Number of Observations
commute time <10 minutes 1
10 ? commute time < 20 minutes 2
20 ? commute time < 30 minutes 4
30 ? commute time < 40 minutes 9
40 ? commute time < 50 minutes 4

Answer: Taking the midpoints of each observation as the value of the random variable, we get .05 × 5 + .1 × 15 + .2 × 25 + .45 × 35 + .2 × 45 = 31.5 minutes