1993 Sarah was born in 1993. In the year 2023, she turned 30 and celebrated her 30th birthday. Her sister, who was 5 years younger than Sarah, would have been born in 1998. So in 2023, the sister was 25 years old and away studying abroad, unable to attend the birthday party. The key information is that Sarah turned 30 in 2023 and has a younger sister who is 5 years her junior. Subtracting 30 from 2023 gives 1993 as the year Sarah was born. The sister's age of 25 in 2023 confirms that Sarah must have been born in 1993, as subtracting her age from 2023 matches the result of subtracting 30 from 2023. Therefore, based on the facts that Sarah turned 30 in 2023 and has a sister who is 5 years younger, the only possible year for Sarah's birth is 1993. No other year could account for the age difference between Sarah and her sister or the specific ages and birth year indicated. The information provided is clear and sufficient to determine Sarah's birth year with certainty. The math directly links the ages in 2023 to Sarah's birth year of 1993 through straightforward arithmetic subtraction. The sister's age adds an additional data point that independently confirms the calculation based on Sarah's age in 2023.