How many months of the year have 31 days? Jan, Mar, May, Jul, Aug, Oct, Dec. That's 7 months out of 12. Thus, if you were to pick a month from these 12, the chances that you'd pick one that had 31 days would be 7/12, or approx. 0.58.
There are 12 months, 7/12 have 31 days. You would have a 7/12 probability of choosing one with 31 days if it is random.
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