Occam's Razor is a line of reasoning that says the simplest answer is often correct.
A) Fermat's Last Theorem:
In 1630, Fermat wrote in the margin of an old Greek mathematics book that he could demonstrate that no integers (whole numbers) can make the equation x (to the power of n) + y (to the power of n) = z (to the power of n) ... true if n is greater than 2.
B) P vs NP problem (whether P ='s NP or not):
Stephen Cook and Leonid Levin formulated the P (i.e., easy to find) versus NP (i.e., easy to check) problem independently in 1971.