alphabetically sorted number

Answer is forty, but how to get to the answer is important.

First check the numbers from 0 to 19 (in that also you can discard some numbers easily), there is no number in this range which is alphabetically sorted.

now if you see 21, 22 will all end with one, two, three etc., these can not be in sorted order as we have already discarded numbers from 0-9

only the numbers at ten’s position, i.e. twenty, thirty, forty, fifty, sixty can have a chance

so just check them and you will get your answer