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Roman numeral
This space station patch has two Xs. They are Roman numerals that stand for the number 20. Image Credit: NASA
a letter used as a number in the very old Roman number system- I equals 1
- V equals 5
- X equals 10
- L equals 50
- C equals 100
- D equals 500
- M equals 1,000
Sentences:
Some NASA patches have Roman numerals on them.
Until 1969, NASA only used Roman numerals for missions that were a success.