Combinations

Question

My teacher gave us six essay prompts to prepare for an upcoming exam. However, the exam will only have four of the original six listed and from those four we must choose one to write an essay about. What is the least amount of essay prompts I should prepare?

Answer 1

First of all, notice that two of the prompts will not be on the test.

 

So if you prepare for two of them, there is a chance that both of those two will be the two that are not on the test, and you will be left with four prompts that you did not prepare for.

 

If  you prepare for three of them, then two of them may be the omitted ones, but at least one of them will be on the test, and you can choose that one.

 

So if you want to approach the test this way, you have to prepare for at least 3 prompts.