Four digit numbers - Math Central

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Hello!
I would like to ask you how many combinations are possible using 4 digit number.
# # # # (1...9). There is no zero,numbers don't repeat,second and forth are odd . Thanks

Hi Anton,

What you should do is fill in the 2nd place first (5 ways to pick an odd number), now do the fourth place (only 4 choices here - why?); notice that you can therefore fill in these two places in 20 different ways. Now from the remaining 7 numbers, fill in the 1st and 3rd spots.

Penny

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