XXI Roman Numerals

What is XXI in Roman Numerals?

The Roman numeral XXI is equal to the number 21.

Unlike other Roman numerals, such as IX for example, which require the subtractive principle method to be used - where a smaller number is written before a larger one and needs to be subtracted from this larger number – the Roman numeral XXI is a relatively straightforward one to convert. It is only 3 letters long, which just need to all be added together.

The sum to convert the Roman numeral XXI into the number 21 is therefore:

X (10) plus X (10) plus I (1), which equals 21.

Math Questions Containing the Roman Numeral XXI

Why not have a go at the below math questions to test your knowledge of Roman numerals? Each of the questions contain the Roman numeral XXI in them somewhere.

$A math question being asked in a classroom$

1. What is XXI plus VI?

This first Roman numeral math question is a nice straightforward one. We know from above that XXI is 21, so all we need to do is work out what number VI equates to, and then add them together to get the answer.

We know from XXI that the letter I is equal to 1. The Roman numeral V is equal to five. Because the I comes after the bigger value (V in this case), then it is added rather than taken away. So it is V + I, which is 5 + 1, and gives us the answer of 6.

Now all we need to do is add 21 (XXI) and 6 (VI), which gives us the answer: 27 (or XXVII in Roman numerals)

2. What is XXII minus XXI?

If you are new to learning about Roman numerals then this question may at first look complicated, but it is actually very simple.

All we need to do to obtain the solution is first work out XXII which is 22 (X + X + I + I or 10 + 10 + 1 + 1), and then take off XXI which we know to be 21, which gives us the answer which is 1 (or I in Roman numerals).

So, although it looked complicated at the beginning, it wasn’t so scary. It’s a good example of how present-day numbers, in this case 22 minus 21, are much more straightforward than the Roman numeral equivalents!

3. What is XXI multiplied by L?

A multiplication question usually produces a big Roman numeral as an answer, but in this case, as we shall see, it is actually one of the more simple large numbers to write in Roman numerals.

To work out the answer, we need to first know that the Roman numeral L is equal to the number 50. In our question it is simply L on its own, with no other numerals before or after it needing to be added or taken away. So our question is simply 21 multiplied by 50.

You might need your calculator for this one, but the answer is 1,050. So how do we write that in Roman numerals? Is it a big long string of letters that will end up going off the page because it is so large?

Fortunately, the answer is no. The ancient Romans had a numeral for the number 1,000 which was M. So to write the number 1,050 all we need to write is ML, which is the equivalent of 1,000 + 50.

4. What is LXXXIV divided by XXI?

Ouch. This looks like a nasty one to finish. Working out the correct answer will be the easiest part of this math problem; the difficult part will be figuring out what number LXXXIV is!

As with all larger Roman numerals, the best way to tackle it is to split it up into sections. Start at the beginning and write down each letter in number form, and remember to take care if you get to a smaller number before a bigger one, as this will require it to be taken away.

So, in the example of LXXXIV, we would begin by writing out the number for each letter...

50 + 10 + 10 + 10 + ... Uh oh, we reached a 1 (I) which comes before a 5 (V). No problem, for this last one we need to take 1 away from 5, which is 4.

So, the equation becomes 50 + 10 + 10 + 10 + 4. Add them all together and we get 84. See, it wasn’t so bad after all.

Now that we have worked this out, we just need to divide 84 by 21, and this gives us the answer to the question, which is 4 (or IV in Roman numerals).