The Romans wrote numbers with letters. I is 1, V is 5, X is 10. Several signs next to each other are added up, so XVII is 10 + 5 + 1 + 1, which makes 17.
A smaller sign BEFORE a bigger one gets subtracted. That is why IV is not 6 but 5 − 1 = 4. In the same way IX is 9. Look carefully at the order the signs come in.
Read both numbers, work it out as usual and write the result back into the tablet in Roman numerals.
Example
For parents
What does this exercise practice?
The stone tablet separates reading a number from its value. Children learn that the same amount can be written down in quite different ways, and practice addition and subtraction up to 20 along the way.
Common pitfalls
The most common mistake is reading IV as 6. It helps to ask whether the small sign is in front or behind (in front means subtract, behind means add). A second stumbling block, there is no Roman sign for zero, which is why no tablet ever works out to zero.
How to practice at home
Look for Roman numerals in everyday life, on tower clock faces, on the house numbers of old buildings or in book chapters. Read together which number is written there.
Related: Secret message