To add two digit numbers with regrouping, follow the following steps:

Example: Add 66 and 27

You want to add 66 + 27

Write the numbers in vertical format

1

6 6

2 7

______________

9 3

Add the ones that is 6 and 7 to get 13.

Then, put 3 in the ones place and carry over 1 to the tens place.

Add the tens that is 1, 6 and 2 to get 9 and put the answer in the tens place

The answer is 93.