The answer should be 18cm
Here is why I think that:
On the ruler, you should count from 9 to 26
9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26.
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
The total tick marks from 9 to 26 is = to 18
(I know that R is resting above the number eight, do not count that one, many people make that mistake)
Please let me know in the comments if
somehow my calculation is wrong.