100 consist of 3 digits [1,0,0] then the result is 1!+0!+0! = 1 + 1 + 1 = 3. The same for 123 -> [1,2,3] = 1! + 2! + 3! = 1 + 12 + 12*3 = 1 + 2 + 6 = 9
task.the-factorials-of-the-digits-of-number