Explanation:
for this you take the midpoints of every interval, multiply them with the corresponding frequency, sum this all up, and divide the result by the total number of cases (here 31 for the 31 days in December).
2 × 12 = 24
7 × 8 = 56
12 × 7 = 84
17 × 4 = 72
--------
236
236 / 31 = 7.612903226... ≈ 7.6
there were estimated 7.6 accidents per day.