Statistics · Hard problem

A histogram shows the ages of $20$ museum visitors: ages $0$–$9$: $4$ visitors; $10$–$19$: $6$ visitors; $20$–$29$: $5$ visitors; $30$–$39$: $5$ visitors. Using the midpoint of each age group ($4.5, 14.5, 24.5, 34.5$) to estimate, what is the estimated mean age?