I'm a bit hesitant to provide you with a complete working solution, since you mention it is homework. I will give you some advice in addition to my first comment:
Use a counter to check if you still have to check the next value in your vector (in your while condition). Use a second counter to keep track of values greater than your threshold.
What while X>24 does is to check the entire vector X against 24, and then that logical vector is used as the criterion. I can never remember what Matlab does in these cases, nor do I need to. You should avoid using an entire vector as a test. Always use the any or all functions to convert them to a scalar. You will (almost) never mean to use a vector as a conditional.
In your case, the while loop immediately exits, which results in the output answer being 0. If Matlab would enter the loop, it would continue looping, since the criterion is not affected by the loop contents.