The question is: "How many months of the year have 28 days?" Maansi quickly answers…

Why

Maansi answered the question she is USED to hearing — 'which month has exactly 28 days' — rather than the question actually asked, which is 'how many months HAVE 28 days', without the word 'exactly' or 'only' anywhere in it. A month with 30 or 31 days still, technically and completely truthfully, HAS 28 days within it — those 28 days are simply followed by 2 or 3 more. The question is not asking which month stops at 28, it is asking how many months contain at least that many days, and every single month, without exception, contains at least 28 days. Test it: read it once more, slowly, exactly as written — 'how many months have 28 days' is genuinely answered by 12, and the trick only works because most people, like Maansi, mentally add a word ('only' or 'exactly') that was never actually there. Her brother is right: all 12 months have at least 28 days — reading the exact words of a question, not the question you expect to hear, is the whole trick here.