An inverse of something is the opposite of it. For example, the inverse of twenty-nine is ninety-two. However, there are other ways to look at inverses. In math, there are inverses as well. An inverse function is one in which the function is reversed.
For example, if there is f of x is equal to y, then the inverse of it is g of y is equal to x. If Henry makes a 65 on the test, then Henry failed the test. However, if you want the inverse of that, you will need to change up both sides of the sentence.
The information before the comma as well as after it should be changed to the reverse or opposite. Therefore, the inverse of it would be if Henry did not make a 65 on the test, then Henry did not fail the test.