Explanation:
the positive integer numbers that are divisible by 7 are an arithmetic sequence by always adding 7 :
a1 = 7
a2 = a1 + 7 = 7+7 = 14
a3 = a2 + 7 = a1 + 7 + 7 = 7 + 2×7 = 21
...
an = a1 + (n-1)×7 = 7 + (n-1)×7 = n×7
the sum of an arithmetic sequence is
n/2 × (2a1 + (n - 1)×d)
with a1 being the first term (in our case 7).
d being the common difference from term to term (in our case 7).
how many terms (what is n) do we need to add ?
we need to find n, where the sequence reaches 200.
200 = n×7
n = 200/7 = 28.57142857...
so, with n = 29 we would get a number higher than 200.
so, n=28 gives us the last number divisible by 7 that is smaller than 200 (28×7 = 196).
the sum of all positive integers below 200 that are divisible by 7 is then
28/2 × (2×7 + 27×7) = 14 × 29×7 = 2,842