To add 7 and 8 step by step, follow these steps:
1. Start with the number 7.
2. Move one unit to the right on the number line to reach 8.
3. Combine both numbers by counting all the units from 7 to 8, which is 1.
4. The sum of 7 and 8 is 1.
Therefore, 7 + 8 = 15.