Here are the combinations:
1x18 = 18
18x1 = 18
9x2 = 18
2x9 = 18
6x3 = 18
3x6 = 18
A subtraction problem can be turned in to an addition problem because of negative numbers. Take 5-2=3 for example. If you change 5-2 into an addition problem then you would but he addition sign and a negative sign on 2. This will then leave you with the problem looking like 5+ -2. If you add 5 and -2 you will still get three.
6.5 times 4 and 13 times 2