How many terms are in the arithmetic sequence 1313, 1616, 1919, ……, 7070, 7373?

To find the number of terms in the arithmetic sequence, we need to use the formula 
   
where  is the nth number,  is the first number, n is the number of terms and d is the difference of the two consecutive numbers.
7373 = 1313 + (n - 1)(303)
7373 = 1313 + 303n - 303
7373 = 1010 + 303n
7373 - 1010 = 303n
6363 = 303n
6363 ÷ 303 = n
n = 21

Therefore, there are 21 terms in the arithmetic sequence given.

The answer would be: b