The graph shows the position of a man walking along a straight path, measured from a gate. What is his velocity between $t = 70\ \text{s}$ and $t = 100\ \text{s}$?

Fig. 1: Graph of a man's position from a gate against time: 0 to 60 m in 40 s, flat at 60 m until 70 s, then down to 30 m at 100 s.