A prescriber writes a prescription for "XCIX tablets." How many tablets did the prescriber order?

“I” equals 1, “X” equals 10, and “C” equals 100. When a lower number appears in front of a higher number, subtract the lower number from the higher number (e.g., “XC” is 100 – 10, or 90, and “IX” is 10 – 1, or 9). Then add those numbers together to equal the total: 90 + 9 = 99.

The prescription for “XCIX tablets” orders:

D. 99

In Roman numerals, XCIX represents 99 (XC = 90 and IX = 9, so 90 + 9 = 99).