Explanation:
the other answer is correct for the readable question, of course, but since the answer options do no fit to that and some strange phrasing in the description lead me to believe that some essential information is not written here.
I suspect it should say something like
the number is added to 1/x of itself.
let's try the different answer options and find what would be the right x :
n = 18
18 + 1/x × 18 = 24
6 = 18/x or x/18 = 1/6
x = 18/6 = 3
so, if we needed to add 1/3 of the number to itself to get 24, the correct answer would be 18.
n = 20
20 + 1/x × 20 = 24
4 = 20/x or x/20 = 1/4
x = 20/4 = 5
so, if we needed to add 1/5 of the number itself to get 24, the correct answer would be 20.
n = 21
21 + 1/x × 21 = 24
3 = 21/x or x/21 = 1/3
x = 21/3 = 7
so, if we needed to add 1/7 of the number itself to get 24, the correct answer would be 21.
n = 233
I think this must be a typo too, but let's just calculate it through.
233 + 1/x × 233 = 24
-209 = 233/x or x/233 = -1/209
x = -233/209 = -1.114832536...
so, no, this is definitely not a homework or test question.
also possible would be 16 (when adding half of the number to itself to get 24).
and 22 (when adding 1/11 of the number to itself to get 24).