The history of Roman math, in general, is poorly documented. This is surprising as the Romans kept amazing records. Everything from population numbers to attendance figures at the Coliseum which was capable of seating fifty thousand spectators. All these records were of course kept using Roman Numerals
Some people speculate that when counting originally became a part of human life the fingers were the first tool to be used to calculate. Five fingers held up would indicate 5. If you hold up your hand and spread your fingers you will notice the obvious V shape between your thumb and first finger. People believe this is how V was chosen to represent 5.
Because the Romans are famous for using tally marks in their documentation, crossing out every 10 tally marks for easy identification; it is thought that the X became a ?short hand? version to represent 10.
C represents 100 from the Latin word centum, meaning 100. Think Century, or 100 cents in a dollar, or even 100 centimeters in a meter.
L represents 50. This letter is in fact a result of several symbols ?evolving? the pre-L symbol looked so like the letter L, that the L was indeed adopted to represent 50. M came from the Latin word mili meaning 1000. This is just a brief history of roman numerals, and how they came to be.