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Roman Numerals

Number · Year 4–Year 6

Build a numeral from the seven symbols. The board reads it back.

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About the roman numerals

The seven symbols are on the toolbar and the numeral is built on the board. Tap X, L, I, X and the board draws XLIX, breaks it into its parts, adds them up and says what it comes to.

The subtractive pair is what a class gets wrong, so it is marked. Read letter by letter, XIV comes out as sixteen: the I in front of the V takes away instead of adding. Every pair that takes away is drawn in its own colour and underlined, in the numeral and again in the sum underneath, so the four in 10 + 4 is visibly the IV rather than a number that appeared out of nowhere.

Nothing is marked wrong. A numeral that breaks a rule is still read out and still worth what it is worth, because “IIII is worth four, and four is written IV” is the teaching moment and refusing to read it is not. The board names the rule instead: no more than three of the same in a row, V and L and D are never repeated, I only goes before V or X, and the parts are written largest first.

The tool works the numeral out from the thirteen steps of the conversion, and every number in the bank is also authored with the numeral it should come to. A test holds the two against each other, so a wrong numeral would fail the build rather than reach a class.

Hide the numbers and the tool turns round. There is no target and no sum, so a teacher can build MDCLXVI and ask the class what it says before showing them, which is the reading half of the objective.

How to use it on your whiteboard

  1. 1Read the number the board is asking for.
  2. 2Tap the symbols to build the numeral. They go on in the order you tap them.
  3. 3Watch the sum underneath: the parts are added up as you go.
  4. 4The target turns gold when the numeral is right.
  5. 5Undo takes off the last symbol, or Clear takes the lot off.
  6. 6Tap Next for the next number, or Hide the numbers and ask the class to read a numeral you build.

Questions

Why is 49 not IL?
Because a symbol can only be taken away from the next two sizes up: I goes before V or X, X before L or C, and C before D or M. So 49 is 40 and 9 written separately, XL and IX, which is XLIX.
How high does it go?
3999, which is MMMCMXCIX. Four thousand needs a bar over the M to mean a thousand times it, and that is not something a primary class meets or something you can tap out of seven symbols.
Is there a zero?
No, and that is worth saying to a class. Roman numerals have no zero and no place value, so the position of a symbol says nothing about what it is worth: the value is the parts added up. That is the difference between their system and ours.
Can the class read numerals as well as write them?
Yes. Tap Hide and the target and the sum come off the board, so you can build a numeral and ask the class what it says. 1666, the year of the Great Fire, is MDCLXVI, which uses six of the seven symbols once each in order.