# formulating this problem..

• Oct 17th 2009, 02:03 AM
transgalactic
formulating 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

$\frac{\binom{5}{1}\cdot \binom{4}{1}}{\binom{10}{2}}=\frac{2}{9}$

is it a correct expression?
• Oct 17th 2009, 02:37 AM
CaptainBlack
Quote:

Originally Posted by transgalactic
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

$\frac{\binom{5}{1}\cdot \binom{4}{1}}{\binom{10}{2}}=\frac{2}{9}$

is it a correct expression?

If they are both odd, there are 20 numbers to choose from, only one of which is correct, so the probability is ..

CB
• Oct 17th 2009, 02:44 AM
transgalactic
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