1. **State the problem:** We need to find the total mass of the staircase given that the bottom step has a mass of 70 kg and there are 10 steps in total. 2. **Analyze the problem:** The staircase has 10 steps, and the mass of each step likely changes in a pattern. Since the bottom step is 70 kg, we need to determine if the mass of each step is the same or varies. 3. **Assumption:** If the steps are identical in size and material, each step has the same mass of 70 kg. 4. **Calculate total mass:** Total mass $= 10 \times 70 = 700$ kg. 5. **Convert to tonnes:** Since 1 tonne = 1000 kg, total mass in tonnes $= \frac{700}{1000} = 0.7$ tonnes. **Final answer:** The mass of the whole staircase is $0.7$ tonnes.