How many two-digit numbers are there such that all two digits are different and the first digit is not zero?
Choices:
A) 50
B) 64
C) 72
D) 79
E) 81
All two digits are different and first digit is not zero.   So first digit can be filled in 9 ways.   And, second digit can be filled in 8 ways.   Total ways = 9*8= 
<gadget id="calculator">9*8</gadget>
<output>72</output>
72   Hence option (72).
<result>C</result>
How many two-digit numbers are there such that all two digits are different and the first digit is not zero?