The graph shows a commuter's journey to work. His journey consists of two stages of travelling, separated by a break of 30 minutes. a) What was his speed (in km/h) during the first stage of his journey? b) What was his speed (in km/h) during the second stage of his journey?

The first stage of the journey starts at $07:00$ with $0 \text{ km}$ distance and ends at $07:30$ with $4 \text{ km}$ distance.

Distance covered ($\Delta d$) = $4 \text{ km} - 0 \text{ km} = 4 \text{ km}$
Time taken ($\Delta t$) = $07:30 - 07:00 = 30 \text{ minutes}$

The speed during the first stage was $\boxed{\text{8 km/h}}$.

The journey has a break from $07:30$ to $08:00$. The second stage of travelling starts at $08:00$ with $4 \text{ km}$ distance and ends at $08:30$ with $9 \text{ km}$ distance.

Distance covered ($\Delta d$) = $9 \text{ km} - 4 \text{ km} = 5 \text{ km}$
Time taken ($\Delta t$) = $08:30 - 08:00 = 30 \text{ minutes}$

The speed during the second stage was $\boxed{\text{10 km/h}}$.