8 pm to midnight (12 pm) = 4 hrs
so the temp dropped -16.8 in 4 hrs (because the temp started at 0)
-16.8/4 = -4.2....so it dropped 4.2 every hr.
temp at 9 pm (1 hr after 8) would be (0 - 4.2) = -4.2
You can state the qurstion in an other way
how many integers between 1-100 has 3 in their prime compostion or how many many numbers can be written as 3*a where a is any number
the multiplication table of 3 shows that 3*33=99
which means that highest product below 100 is 3*33
so there are 33 numbers that are divisible by three