A fence is 153 cm long. It has 3 posts, each 15 cm wide. How long is each gap between the posts?

$$\text{Total width of posts} = \text{Number of posts} \times \text{Width of one post}$$ $$\text{Total width of posts} = 3 \times 15 \text{ cm}$$ $$\text{Total width of posts} = 45 \text{ cm}$$

$$\text{Total length of gaps} = \text{Total length of fence} - \text{Total width of posts}$$ $$\text{Total length of gaps} = 153 \text{ cm} - 45 \text{ cm}$$ $$\text{Total length of gaps} = 108 \text{ cm}$$

Divide the total length of the gaps by the number of gaps to find the length of one gap.

$$\text{Length of one gap} = \frac{\text{Total length of gaps}}{\text{Number of gaps}}$$ $$\text{Length of one gap} = \frac{108 \text{ cm}}{2}$$ $$\text{Length of one gap} = 54 \text{ cm}$$

The length of one gap between two posts is $\text{54 cm}$.