Yep.
a) If a person misses one, he gets 93%
b) I got 93%
Therefore, I missed one.
Valid? Invalid?
If His first condition said Iff (if and only if), then it would be valid. Also, his first statement could have read: "If a person gets a 93%, then he missed one," then it would be valid. Written as it is, however, only said that if you missed one, you get a 93%, and does not take into account other possibilities of wrong answers. It does not take into account other possible ways to achieve a 93%, which it needed to in order to fulfill the first statement.