Roman numerals · Usually met around P3
Roman numeral rules: spotting a wrong one
Read it, write that number properly, compare.
How to do it
Method
Read it, write that number properly, compare
IC → 99 → XCIX: different, so IC is wrong
Another way
Check each rule
IIII: four I's in a row, so it is wrong
In full
Roman numerals keep four rules. One: I, X, C and M repeat at most three times, so 4 is IV, not IIII. Two: V, L and D never repeat, so 10 is X, not VV. Three: only I, X and C go in front to take away, and only before the next two letters up: I before V and X, X before L and C, C before D and M, so 99 is XCIX, not IC. Four: V, L and D never go in front to take away. There are two ways to check a numeral: go through the rules one by one; or read it as it stands, then write that number properly place by place. If the two don't match, the first was written wrongly.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
The numeral on the tablet is written correctly.
True or false?
Show the answer and working
Answer: False
84 is written properly with IV, IX, XL or XC taking away; no letter may appear four times in a row.
Example 2
The numeral on this tablet is written wrongly. What number did the writer mean?
Show the answer and working
Answer: 100
Read it as it stands, left to right, taking away a smaller letter before a bigger one: it means 100. But 100 must be written place by place, not like this.
Example 3
How many of the tablets are written wrongly?
Show the answer and working
Answer: 2
Read each one, then write that number properly and compare: 2 don't match, so they are wrong.
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