**Solution:**

To find the percentage of tin in the alloy, we need to determine the ratio of tin to the total weight of the alloy.

Let's assume the total weight of the alloy is 100 parts.

Given that the alloy consists of 20 parts of tin and 100 parts of copper, we can find the weight of tin and copper in the alloy.

Weight of tin = (20/120) * 100 = 16.67 parts
Weight of copper = (100/120) * 100 = 83.33 parts

Now, we can calculate the percentage of tin in the alloy.

Percentage of tin = (Weight of tin / Total weight of alloy) * 100
= (16.67 / 100) * 100
= 16.67%

Therefore, the percentage of tin in the alloy is 16.67%.

**Explanation:**

1. **Determine the total weight of the alloy:** We assume the total weight of the alloy is 100 parts for easier calculation.

2. **Calculate the weight of tin and copper:** The given ratio states that the alloy consists of 20 parts of tin and 100 parts of copper. To find the actual weight of tin and copper in the alloy, we use the given ratio and multiply it by the assumed total weight of the alloy.

3. **Calculate the percentage of tin:** To find the percentage of tin in the alloy, we divide the weight of tin by the total weight of the alloy and multiply by 100. This gives us the proportion of tin in the alloy as a percentage.

4. **Final answer:** The calculated percentage of tin represents the amount of tin in the alloy out of the total weight. In this case, the percentage of tin is 16.67%.