A1 is a drop down box containing either 'Yes' or 'No'.

In A2,
If A1 is yes, the user would manually key in a number. This number is a variable.
However, if A1 is no, I'd like A2 to auto-populate '0'.

Do note that A1 and A2 are merged cells.

Is this possible? How can I do this? If not possible, anyone have a suggestion for something similar?