What is the sum of all the integers less than 700 that are divisible by 3and 7?

List of all positive integers less than 700 divisible by 3 and 7 is :  21, 42, 63,......, 693
Thus the A.P. has a = 21 ;   d = 42 - 21 = 21 ; a_n = 693
Now,    a_n = a + (n - 1)d
            693 = 21 + (n - 1)(21)
            672 = (n - 1)(21)
            n - 1 = 32
            n = 33
Now,   S_n = (n/2)(a + a_n)
            S_n = (33/2)(21 + 693)
            S_n = (33/2)(714)
            S_n = 33 × 357
            S_n = 11781