If you break up the journey, Jerry covers 5 m in the first 2 s, 10 m in the next 2 s and so on. So, in the first 2 s, the average speed is $$\frac{5 \, \mathrm{m}}{2 \, \mathrm{s}} = 2.5 \, \mathrm{m/s}$$. In the next 2 s, the average speed is is $$\frac{10 \, \mathrm{m}}{2 \, \mathrm{s}} = 5 \, \mathrm{m/s}$$