#1**+1 **

How many different 4-digit numbers have exactly two different digits?

There are 576 four digit numbers that have exactly 2 different digits!

Guest Dec 24, 2020

#3**+1 **

Neither digit is zero, 2 of each one

9C2*4!/(2!2!) = 9C2* 6 = 36*6 = 216

Zero plus one other number, 2 of each

9*3!/2! = 27

neither digit is zero, 3 of one and 1 of the other.

9C2*2*4 = 36*8 = 288

One digit is zero, 3 of one and 1 of the other.

9*3 = 27

216+27+288+27 = 558

Note: There is room here for error.

Melody Dec 29, 2020