premise: Thanks to the thirty Heads who responded to my question about maternity leave. Clearly, the amount of maternity leave offered to teachers at international schools varies greatly from school to school.
hypothesis: The number of Heads that responded to the question was thirty nine minus ten.
reason: The number of heads that responded to the question was thirty, thirty nine minus ten makes it twenty nine heads, making the statement definitely incorrect. It seems that the system has some problem with math.