statistics question

The following Table 1 shows the durations of 40 telephone calls from an office via the office switchboard: Table 1 Duration (Minutes) 0- 1 1 - 2 2 -3 3-5 5 - 10 Number of Calls 6 10 15 5 4 (a) Estimate the sample mean duration. [3] (b) Estimate the sample variance of durations. [3]