For an estimation, you could truncate to 1 decimal place and add:
9.3+5.1=14.4
To get a better estimate, add the second decimal place, if it is 5 or more, then round up the estimate. Second decimal place = 2+3=5 >=5, so round up 14.4 to 14.5.
Yep! Now you just neeed to round up until you get to the last decimal place.
What do you think the 5 could be rounded to? up to 6, or down to stay at 5?