If you have trouble adding and subtracting, you will find the math sections of the ASVAB/AFQT to be very troublesome. Not only must you be accurate, but you must also be quick. You should know the answers to simple addition and subtraction questions without even thinking.
Get a stopwatch or a wristwatch with a second hand, and see how long it takes you to answer the following addition and subtraction items.
Add the following.
Subtract the following.
How long did it take you to do these? If ...