Sure, let's solve the problem step-by-step.
a) Find the sum of the greatest number of 4 digits and a one-digit number.
1. The greatest number of 4 digits is 9999.
2. A one-digit number to add in this case is 9.
3. Therefore, we need to find the sum of 9999 and 9.
Adding these two numbers:
```
9999
+ 9
-------
10008
```
So, the sum of the greatest number of 4 digits (9999) and a one-digit number (9) is 10008.
