Most English language learners are familiar with contractions as they relate to words. However, there seems to be a bit of confusion when they are used as numbers.
The key visual component when reading or using a contraction is the apostrophe. In general, apostrophes are used for three things, 1- to show omission, 2- for forming the possessive, and 3- for forming plurals. (An apostrophe should almost never be used to form a plural, but there are some rare exceptions.)
Apostrophes show or indicate missing letters or numbers in the formation of contractions. Example: I'll go to the store with you. The apostrophe shows us that the letters wi are missing in the contraction I'll (formed from I will).
In a similar way, contractions for numbers follow the same rules as contractions for words. Example: Jessica graduated from high school in '05. The apostrophe shows us that the numbers 20 are missing from her graduation year of 2005. Most contractions for numbers should not be used to indicate anything older than 100 years, though. Contractions are also generally used for years in the past, as well.
Have you had any problems or other experiences when using a contraction as a number?