The time in Accra, Ghana along longitude 0° is 7.00am. What is the time in Kigali along longitude 15° East?

Accra is at $0^\circ$ longitude. Kigali is at $15^\circ$ East longitude.
The difference in longitude is:

$$\Delta \text{Longitude} = 15^\circ \text{ East} - 0^\circ = 15^\circ$$

The Earth rotates $360^\circ$ in 24 hours. This means $1^\circ$ of longitude corresponds to:

$$\frac{24 \text{ hours}}{360^\circ} = \frac{1}{15} \text{ hours/}^\circ = 4 \text{ minutes/}^\circ$$

The time difference is:

$$\text{Time difference} = 15^\circ \times 4 \text{ minutes/}^\circ = 60 \text{ minutes} = 1 \text{ hour}$$

Since Kigali is East of Accra, the time in Kigali will be ahead of Accra's time.

$$\text{Time in Kigali} = \text{Time in Accra} + \text{Time difference}$$ $$\text{Time in Kigali} = 7:00 \text{ am} + 1 \text{ hour} = 8:00 \text{ am}$$

The time in Kigali is $\text{8:00 am}$.