The diameter of a circle is 13 m find it’s circumference in terms of pi

You were told that the amount of time lapsed between consecutive trades on the new york stock exchange followed a normal distribution with a mean of 15 seconds. you were also told that the probability that the time lapsed between two consecutive trades to fall between 16 to 17 seconds was 13%. the probability that the time lapsed between two consecutive trades would fall below 13 seconds was 7%. what is the probability that the time lapsed between two consecutive trades will be between 14 and 15 seconds?

Which geometric principle is used to justify the construction below?

Geometry - finding area of regular polygons and sectors (check my math, i can’t find the error)