Solve the following problems: 1. A piece of timber 213 cm 5 mm long is cut into 5 equal pieces. Find the length of each small piece. 2. A bottle is 26 cm 9 mm high. What is its height in millimeters?

We know that $1 \text{ cm} = 10 \text{ mm}$.

$$213 \text{ cm } 5 \text{ mm} = (213 \times 10) \text{ mm} + 5 \text{ mm}$$ $$= 2130 \text{ mm} + 5 \text{ mm}$$ $$= 2135 \text{ mm}$$

The timber is cut into 5 equal pieces.

$$\text{Length of each piece} = \frac{2135 \text{ mm}}{5}$$ $$= 427 \text{ mm}$$

$$427 \text{ mm} = 420 \text{ mm} + 7 \text{ mm}$$ $$= (420 \div 10) \text{ cm} + 7 \text{ mm}$$ $$= 42 \text{ cm } 7 \text{ mm}$$

The length of each small piece was $\boxed{\text{42 cm 7 mm}}$.

2. Height of the bottle in millimeters

We know that $1 \text{ cm} = 10 \text{ mm}$.

$$26 \text{ cm} = 26 \times 10 \text{ mm}$$ $$= 260 \text{ mm}$$

$$\text{Total height} = 260 \text{ mm} + 9 \text{ mm}$$ $$= 269 \text{ mm}$$

The height of the bottle in millimeters was $\boxed{\text{269 mm}}$.

3. Length of the ribbon left

$$31 \text{ cm } 6 \text{ mm} = (31 \times 10) \text{ mm} + 6 \text{ mm}$$ $$= 310 \text{ mm} + 6 \text{ mm}$$ $$= 316 \text{ mm}$$

$$5 \text{ cm } 3 \text{ mm} = (5 \times 10) \text{ mm} + 3 \text{ mm}$$ $$= 50 \text{ mm} + 3 \text{ mm}$$ $$= 53 \text{ mm}$$

$$\text{Length left} = 316 \text{ mm} - 53 \text{ mm}$$ $$= 263 \text{ mm}$$

$$263 \text{ mm} = 260 \text{ mm} + 3 \text{ mm}$$ $$= (260 \div 10) \text{ cm} + 3 \text{ mm}$$ $$= 26 \text{ cm } 3 \text{ mm}$$

The length of the ribbon left was $\boxed{\text{26 cm 3 mm}}$.