[perlgeek]

So you limited yourself to non-negative integers, and had no idea what negative numbers or fractions would do.

[bbcbasic]

Also he didn't know what numbers whose digits add up to 88 would do.

[__david__]

Their input on the phone wouldn't allow minus or period characters so I assumed positive integers were the set.

[eli]

Based on the context of the question and the UI of the testing interface, fractions seem unlikely to be an intended part of the question. I likewise wouldn't bother testing unicode U+216x roman numerals.

[Ntrails]

It refused to accept both fractions and imaginary numbers. I did test negatives and zeros since that was really the only remaining set I could think of.

Most importantly I used about 6 tests (3 right 3 wrong) to come up with the answer and then did another 17 looking for the trick. After all, it couldn't just be that simple right?

[ubernostrum]

The presentation of the problem -- and I know that trusting the problem state is unwise sometimes in cognitive-bias tests, since many such tests are actually designed to be "we said we were asking X but actually meant Y" -- indicated a simple rule, rather than one which would behave differently on different classes of numbers.

So after the tests listed above I felt confident enough to guess.