Question

Find the sum of all the numbers that can be formed with the digits
2, 3, 4, 5 taken all at a time.

Answer 1

for each digit in each spot, if we fix that digit , there are 3! way to arrange the four numbers with that digit fix .so in sum we get 2*3! + 3*3! + 3*4! + 3*5! = 84
so for each position tens, hundred, thousands sum of units digits will be equal hence sum = 84*1000 + 84*100 + 84*10 + 84 = 93324