Question

before prof. ribleys challenging, calculus exam, everyone hoped to make a passing grade. after the test, however, all of the students prayed for at least a 50.

no change is necessary.
add a comma between hoped and to.
remove the comma between however and all.
remove the comma between challenging and calculus.

Explanation:

1. The comma between "challenging" and "calculus" is incorrect because coordinate adjectives (adjectives that independently describe the noun "exam") do not need a comma when the order can be reversed and "and" can be inserted between them without changing meaning ("calculus challenging exam" is not standard, but more importantly, "challenging" directly modifies the specific type of exam, "calculus exam," so they function as a single unit).
2. The phrase "hoped to" is a standard verb-infinitive structure that does not require a comma.
3. The phrase "however, all" correctly uses a comma after "however" when it interrupts the sentence, so that comma should stay.

Answer:

Remove the comma between challenging and calculus.