first digit time second digit and second digit times first digit then repeat
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There are 3 values (1, 2, 3) for each of the 4 digits. Therefore, there are 3*3*3*3 or 81 four digit numbers that can be formed.
If you can repeat a digit, there are 27. If you can't repeat a digit, there are only 6.
60. There are 5 different choices for the first digit, four for the second digit, and three choices for the third digit. According to the fundamental counting principle, you multiply them together to get the total number of possible ways, and 5 x 4 x 3 = 60 Another way to think about it is 5!/3!
There are no two real numbers that will add to -9 and multiply to 81. Using complex numbers, the two numbers are: (-9/2 + i9/2√3) and (-9/2 - i9/2√3)
The final digit must be a 2 to form an even number. The first digit may be any of three remaining digits (1, 3, 5) while the second digit may be any of the two remaining digits. All together, that makes 3*2 = 6 distinct even numbers.