<s>[INST] What is the sum of numbers between 250 and 350 which are divisible by 7? 
What is the sum of numbers between $250$ and $350$ which are divisible by $7$?

My attempted solution:
7|250|35
 |21 |
 -----
   40
   35
  ----
    5

The 1st number divisible by $7$ is: $(250-5)+7 = 252$. 
7|350|50
 |35 |
 -----
    0

The last number divisible by $7$ is: $350$.
Total numbers between 250 and 350 divisible by $7$ is: 
$((350-252)/7) + 1$
$ = (98/7)+1$ 
$ = 14+1$
$ = 15$
$ Sum = \frac{15}{2} [2 \cdot 252 + (15 - 1) \cdot 7 ]$ 
$ = \frac{15}{2} [2 \cdot 252 + 14 \cdot 7 ]$
$ = \frac{15}{2} [504 + 98 ]$
$ = \frac{15}{2} \cdot 602$
$ = 15 \cdot 301$
$ = 4515 $ 
(Ans.) $4515$
Is there a more efficient way to do this?
 [/INST] HINT
The first divisible number is $252$ the last $350$ then
$$252+\dots+350=7(36+\dots+50)=7\left(\sum_{k=1}^{50}k-\sum_{k=1}^{35}k\right)$$
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