Remember that using Roman Numerals each symbol has a value and you add or take away the values according to these rules:

• Add the value if  it is larger than the value of the symbol to the right of it.

• Take away the value if it is smaller than the value of the symbol to the right of it.

ie CIXVI = 100 - 1 + 10 + 5 +1

you take away the I(1) because it is smaller in value to the X(10) to the right of it.