How many natural numbers between 17 and 80 are divisible by 6?
Answer: B
The first multiple of 6 after 17 is 18 (6x3). The last multiple of 6 before 80 is 78 (6x13).
The numbers form an A.P.: 18, 24, ..., 78.
The number of terms can be found by the indices of the multiples: 3, 4, ..., 13.
Number of terms = (Last index - First index) + 1 = (13 - 3) + 1 = 11.