The first thing to do is make both fractions over 6:
1/3 = 2/6
Sue at 2/6 of the crackers.
Now we can add the fractions:
2/6 + 1/6 = 3/6
The next thing to do is cancel down the fraction.
3/3 = 1
6/3 = 2
3/6 = 1/2
Now that we know that half of the crackers have been eaten, we halve the total amount:
18 X 1/2 = 18 / 2 = 9
Bob had 9 crackers left.