Seven.
I use the old rhyme to remember -
30 days hath September,
April, June and November
All the rest have 31
Excepting February alone,
Which has but 28 days clear
Or 29 in each Leap Year.
Only February has exactly 28 days in a non leap year.
About 6 months (exactly 6 months in a leap year of 366 days). There is no fixed number of days in a month, but the average is 30.4 days a month.
All 12 months have "at least" 28 days. Only February has exactly 28 days, except in leap year.
11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.
If by "only 31 days" you mean "exactly 31 days", then 7. If by "only 31 days" you mean "not more than 31 days" then all of them.
Only February has exactly 28 days in a non leap year.
Almost exactly 3 months for a year of 365 days. (2.998 average months) An average month is about 30.4 days because months can vary in the number of days. Exactly 3 months is 91.3 days in a Julian year, either 91.25 or 91.5 calendar days
12 months exactly. There are always 12 months in a year of 365 days (366 in leap year)
About 6 months (exactly 6 months in a leap year of 366 days). There is no fixed number of days in a month, but the average is 30.4 days a month.
91.3125 (leap year taken into account)
Exactly 48 months. The period 1461 days is exactly 4 years (three years and one leap year). In this case, you do not have to use the average for days in a month (30.4 days).
All 12 months have "at least" 28 days. Only February has exactly 28 days, except in leap year.
11. The question is not specifically asking how many months have "exactly" 30 days. 11 months have "at least" 30 days. There are four months that do have exactly 30 days, and there are at least five different versions of this riddle.
465 days is about 15 months (15.28 months, about 15 months and 8 days). There is no fixed number of days per month, although there are exactly 12 per year for an average of 30.4 days per month.
The riddle answer is "12" because all 12 months contain "at least" 28 days. February is the only month that has 28 days exactly, but 29 in a leap year.(The trick is that everyone says "1" because of this distinction. The version "If there are 7 months that have 31 days and 11 months that have 30 days, how many months have 28 days?" telegraphs the answer unless someone doesn't know how many months there are in total. So it is either erroneous or trivial.)
11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.11 months of the year have at least 30 days.
If by "only 31 days" you mean "exactly 31 days", then 7. If by "only 31 days" you mean "not more than 31 days" then all of them.