Which one of the following equals the number of multiples of 3 between 102 and 210, inclusive?

The numbers 102 and 210 are themselves multiples of 3.
also a multiple of 3 exists once in every three consecutive integers.
Counting the multiples of 3 starting with 1 for 102,
2 {= 1 + (105 - 102)/3 = 1+ 1 = 2 } for 105,
3 { = 1 +(108 - 102)/3 = 1 + 2 = 3} for 108,
and so on, the count we get for 210 equals 1+(210 - 102)/3 = 1 + 36 = 37,
hence the answer is 37