posted by Ann on .
How many odd four-digit numbers, all of the digit different can be formed from the digits 0 to 7, if there must be a 4 in the number?
There are four options for the last digit (1, 3, 5, or 7). One of the other three digits only has 1 option (for the 4). One of the other remaining digits will have 6 options and the final digit will have 5 options (because they can't repeat the first two) Therefore, the answer is:
I do not have much confidence in my answer. Could someone please tell me if I did this right? Thanks for your help.
The "4" could be in
the first place,
the 2nd place, or
the 3rd place
so you could have 4 _ _ (odd)
_ 4 _ (odd)
_ _ 4 (odd)
the first of these could be done
1 x 5 x 4 x 4 = 80
5 x 1 x 4 x 4 = 80
5 x 4 x 1 x 4 = 80
for a total of 240
explanation of the first case:
1. fill in the 4 possiblities to make it odd, at the far right
2. fill in the "4" at the front, 1 way only
3. two digits have now been used up, leaving 5 of the remaining to go in the second spot.
4. three digits have now been used, leaving 4 of the remaining to go in the third spot.
thus : 1x5x4x4
Many of these kind of questions can be easily done by splitting them up into "cases"
Thanks but I'm stumped on part 3 of the explanation for the first case. We have used up 2 digits when we move on to the second space but didn't we start with 8 digits (0-7) so I don't understand why we aren't left with 6 choices for the second spot (8-2). Please help me understand this.