what is the sum of all the two-digits numbers which when divided by 7 gives a remainder of 3?

According to question,
First Two Digit Number, which is divided by 7 and give the remainder 3, will be 10.
Last Two Digit Number, which is divided by 7 and give the remainder 3, will be 94.
The common difference between two consecutive numbers will be 7.
This series will be like ?10, 17, 21 ,............................ 94.
Here First Number a = 10, Last number t_n = 94, Common Difference d = 7 ;
Using the Formula for Last Number t_n = a + (n - 1) x d;
? a + (n - 1) x d = t_n
Put the value of a , d and t_n in above equation.
? 10 + (n - 1) x 7 = 94
? 10 + 7n - 7 = 94
? 7n + 3 = 94
? 7n = 94 - 3 = 91
? n = 91/7
? n = 13
Using the formula for the sum of Arithmetic Progression.
S_n = n/2 [ First Number + Last Number ] ;
Put the value of n , First Number and Last Number, we will get,
S_n = 13/2 [ 10 + 94 ]
S_n = 104 x 13/2
S_n = 52 x 13
S_n = 676