We roll 3 dice and add the results. what is the probability that the total is NOT 18?

To answer this you need to find the probability that it is 18 first. The only way to get 18 is by rolling 3 6's. The probability of each 6 is 1/6. 1/6 * 1/6 * 1/6 = 0.0046. That is the probability of rolling 3 die and getting 18. To get the actual answer, you just subtract that from 1

1-0.0046 equals 0.9954