From 15 to 75 inclusive, there are 75 - 14 = 61 integer numbers. There are 20 gaps of the length 3 between 15 and 75; hence, the number of integers in this interval [15,75] that are multiple of 3 is 20 + 1 = 21. The numbers that are not divisible by 3 are the rest 61 - 21 = 40 numbers. ANSWER