The probability of a particular day being a unique birthday is 365/365. For a second person to have a birthday that doesn't match that of the first, he or she must be born on any one of the other 364 days of the year. To obtain the probability of no match between two people multiply:
365/365 times 364/365 which = .9973.
Between three people:
365/365 times 364/365 times 363/365 which = .9918.
As you continue along this path, you'll find that at 23 people the odds of no match are .4927, therefore the chance of at least one match within a group of 23 people is:
1 - .4927 = .5073, or slightly better than 50 percent.

Converting a Fraction to a Decimal
To convert a fraction to a decimal, do the following
For example: Convert 3/5 to a decimal.
Divide the numerator of the fraction by the denominator
(e.g. 3 ÷ 5 = 0.600)
Round the answer to the desired precision.

More links to converting Fractions to Decimals
...... WebMath.com
...... mathsisfun

Online Fraction Calculator
... helpwithfractions.com
... algebrahelp.com
Converting Fractions Calc
... csgnetwork.com