The number 15 is written as XV in Roman numerals. The symbol X stands for ten while the symbol V stands for five. The symbol for the smaller number must be written after the symbol for the larger number.
One of the basic rules for writing numbers in Roman Numerals is that only seven symbols can be used and no more. These seven symbols are I, V, X, L, C, D, and M. It is also important to understand that when a symbol for a smaller number is written before a symbol for a larger number, it is subtracted from the larger number. If a symbol for a smaller number is written after a symbol for a symbol for a larger number, it is to be added to the symbol for the larger number.