a man forgot the last 2 digits of a phone number.

he only remembers that they are different

and they are odd

what is the chance that he will pick the correct combination of numbers

of the original phone number

is it a correct expression?

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- Oct 17th 2009, 02:03 AMtransgalacticformulating this problem..
a man forgot the last 2 digits of a phone number.

he only remembers that they are different

and they are odd

what is the chance that he will pick the correct combination of numbers

of the original phone number

is it a correct expression? - Oct 17th 2009, 02:37 AMCaptainBlack
- Oct 17th 2009, 02:44 AMtransgalactic
5 posibilities for first

4 forthe second

why 20 numbers to choose from

the number are 0..9

The digits are odd so we have {1, 3, 5, 7, 9} to choose them from. The first can be any of these five and the second any of the remaining 4 (since the digits are different) - so total is 5x4=20

CB