How is using mental math to add with decimals like using mental math to add whole numbers? how is it different?

Well you use the same process really. Taking the individual numbers, lining them up and adding them. However, it is much more difficult because there is more to remember and you have to carry over more numbers. 

When adding these different types of numbers, you use the same process.  The only difference is you need to keep track of the decimal spot, and make sure you don't place the decimal in the wrong position.