4.4 in a hypothetical seismically active region, earthquakes have been recorded over an 80-year period. part of the record is instrumental, but part is not. combining all available data, it appears that the earthquakes have been distributed as follows:

moment magnitude / number of earthquakes

3-4 / 1800

4-5 / 150

5-6 / 11

> 6 / 1

(a) estimate the gutenberg-richter parameters for the region.

(b) neglecting earthquakes of magnitude less than 3, compute the probability that an earthquake in the region will have a moment magnitude between 5.5 and 6.5,

(c) repeat part (b) assuming that paleoseismic evidence indicates that the region is not capable of producing earthquakes of moment magnitude greater than 6.5

i am wondering how to complete part b? i see the solution here on chegg, but i do not understand where the writer came up with the equation?