Solve Addition Problem: Adding 5 Units to 15

The problem requires us to add 5 units to the number 15. To solve this, we will use simple addition. Here is the step-by-step process:

• Step 1: Identify the numbers we are working with. We have the number 15.
• Step 2: We need to add 5 to 15.
• Step 3: Perform the addition: $15 + 5$.

This addition can be computed as follows:
- Start from 15, and count 5 numbers forward: 16, 17, 18, 19, and finally 20.
Thus, $15 + 5 = 20$.

Therefore, the number we get after adding 5 units to 15 is $20$.

The answer matches choice 3.