Free interactive math game: To Arabic — to 20. Part of the Roman Numerals category on MathKite.
Taken from the generator that powers this exercise. A fresh set appears every time the page is opened, and the printable worksheet holds ten of them.
Reading a Roman numeral as an ordinary number.
Add the values from left to right, but subtract any symbol that is smaller than the one immediately after it.
XLII is 50 minus 10 for XL, plus 1 and 1, giving 42.
Adding straight through. Reading XL as 60 instead of 40 misses the subtraction rule.