Data and Displays · Superchallenge problem

Waiting times are shown in a correctly drawn histogram with unequal bins (area $=$ count, height in people per minute): $0$–$10$ min height $1.2$; $10$–$15$ min height $4$; $15$–$25$ min height $1.8$; $25$–$45$ min height $0.4$. Assuming the times are spread evenly within each bin, estimate the median waiting time in minutes.