1. **State the problem:** We are given the length of a virus as 60 nanometres and need to convert this length into millimetres. 2. **Recall the conversion factors:** - 1 metre = 1,000,000,000 nanometres - 1 metre = 1,000 millimetres 3. **Convert nanometres to metres:** $$60 \text{ nanometres} = \frac{60}{1,000,000,000} \text{ metres} = 6 \times 10^{-8} \text{ metres}$$ 4. **Convert metres to millimetres:** $$6 \times 10^{-8} \text{ metres} = 6 \times 10^{-8} \times 1,000 \text{ millimetres} = 6 \times 10^{-5} \text{ millimetres}$$ 5. **Answer for part (a):** The length of the virus in millimetres is $$6 \times 10^{-5}$$ millimetres. 6. **Answer for part (b):** Nanometres are more appropriate units for writing the length of the virus because the virus is extremely small, and using millimetres results in a very tiny decimal number which is less convenient and less precise for such small measurements.