Outside a home there is a keypad that will open the garage if the correct four digit code is entered.? How many codes are possible?

Assume that none of the digits on the keypad are repeated. This means that we have 10 digits: 0,1,2,3,4,5,6,7,8,9.

The number of ways to arrange 10 distinct digits is

10! = 10*9*8*7*6*5*4*3*2*1 = 3,628,800

Answer: 3,628,800

That is 3 million, 628 thousand, and 800.