what is the units digit sum of 1! + 2! + 3! + .....+n! when n = 2019?
Notice that after 5! every number will end in 0. So, we are just looking for 1!+2!+3!+4! which ends in a 3.
In 5! and up, there is a 2 and a 5, and when multiplied, they equal 10, which has a 0 in the units digit.
+4609 
+773 
