So with people who do order of operations incorrectly, you have 25-5 =20 and then divide it by 5 and you get 4.

However the correct answer is 24 since you do the division first.

Where the joke comes in is he states “4!”, which sounds like he is emphatic about the wrong answer. However an exclamation mark is “factorial” which translates to 4x3x2x1… which happens to equal the correct answer of 24.