What is the unit digit of $1! + 2! + 3!$ + … + 50!?

Explanation

1. 01

   From 5! onwards every factorial contains $2 \times 5$, so it ends in 0.

2. 02

   Only $1! + 2! + 3! + 4! = 1 + 2 + 6 + 24 = 33$ affect the unit digit.

3. 03

   So the unit digit is 3.