A better question would be: what percentage understand what a random variable is and will be able to apply this understanding later. See http://www.youtube.com/watch?v=WwslBPj8GgI for exploration of the difference by a Harvard physics professor who realized a few years into his teaching career that most of his into physics students weren't learning very much, although they were able to apply all the appropriate mathematical recipes to all the standard type homework and exam questions.