How do you ADD decimals? First, add the 3 and 6, get 9. That's to the right of the decimal. Nothing to carry.
18.3
29.6
------
.9
Now add the 8 and 9. You get 17. Put down the 7 carry the one to the next column.
118.3
29.6
--------
7.9
Now add 1 and 2 plus the extra 1 you carried. 1+1+2=4. Answer is 47.9
118.3
29.6
--------
47.9