Question

How many 4-digit odd numbers can you make using the digits 1 to 7 if the numbers must be less than 6000? No digits are repeated.

Answer 1

Step-by-step explanation:he first digit must be a 6, 7, or 8, because the number has to be bigger than 6,000.

The last digit must be a 1, 3, 5, or 7, because the number has to be odd.

So if the first digit is 6 or 8, there are 4 odd-number options for the last digit. If the first digit is a 7, then there are only 3 options for the last digit, as 7 can’t be used twice.

The other two digits can be anything that’s not already used. So for each first-and-last combination of digits mentioned above, there will be six options available for one of the middle digits, and then for each of those 3-digit combinations, five options will be available for the final remaining digit.

At this point, we have everything we need to express this as an equation:

x = (2 x 4 + 1 x 3) x 6 x 5

And then we can simplify and solve:

x = (8 + 3) x 30

x = 11 x 30

x = 330