if no digit can be used more than once in a number, how many 2-digit numbers can be formed using only the digits 7, 1, 3, and 9?
a 2
b 6
c 8
d 10
e 12
To find the number of 2-digit numbers that can be formed, we need to consider the number of choices for each digit in the tens place and the units place separately.
For the tens place, we have 4 options (7, 1, 3, or 9).
For the units place, we have 3 options (since we cannot reuse the digit chosen for the tens place).
Therefore, the total number of 2-digit numbers that can be formed is 4 choices for the tens place multiplied by 3 choices for the units place, which is equal to 4 * 3 = 12.
So the answer is e) 12.