Data & Statistics · Challenge problem

A histogram of travel times has four bars: $0$ to $10$ minutes (frequency $4$), $10$ to $20$ minutes (frequency $8$), $20$ to $30$ minutes (frequency $8$), and $30$ to $40$ minutes (frequency $x$). Using the midpoint of each interval to estimate the mean, the estimated mean travel time is exactly $20$ minutes. Find $x$.