Add solution to project euler problem 20

Project Euler/Problem 20/problem20_sol1.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# n! means n x (n - 1) x ... x 3 x 2 x 1
+
+# For example, 10! = 10 x 9 x ... x 3 x 2 x 1 = 3628800,
+# and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
+#
+# Find the sum of the digits in the number 100!
+
+# method to calculate factorial of a number
+def factorial(number)
+  number.downto(1).reduce(:*)
+end
+
+# fetch digits of factorial of `number` and find
+# sum of all those digits, and prints the result on the console
+number = 100
+puts factorial(number).digits.sum