Find a number ~ n ~ (where 10<=n<=20) for which n is two times the sum of that number's digits. Is your answer the only possible solution?
The meaning of '10<=n<=20' is 'n could be equal to 10, or n could be greater than 10, or n could be less than 20, or n could be equal to 20'. This is a fine example of mathematicians inventing a short code to cut down on the number of words needed to explain something. Another alternative is to say 'n could be 10 or 11 or 12 or 13 or 14 or 15 or 16 or 17 or 18 or 19 or 20'. Which could be shortened to 'n could be 10, 11, 12....18,19,20'.
Let the number n be represented by a and b where a, b are the two digits that make up n. For example ~ if a=1 and b=4 then n is 14. It is also possible that a=b.
Do not despair. There are only 11 possible answers. Part of the task is ~ possibly ~ find a short cut. (That is what mathematicians try to do ~ to cut down on laborious stuff.)