How many numbers between 10 and 99 inclusive contain different digits? (i.e. 36 has different digits but 33 doesn't)

Question

How many different numbers can you make if you use the digits 1, 2, 3, 4 once each, in any order, and a times sign. For instance 31x24, or 2x413.

Answer 1

81

---

1,2 to 99- 99 of total numbers
1, 2 to 9- 9 of 1-digit numbers
11, 22 to 99- 9 of double same digit numbers
99-9-9=81 is the answer