Вот решение:
#include <vector>
#include <iostream>
int main () {
// Better to initilize the vector like this instead of using multiple push_back
std::vector<int> a;
for (int i=10; i<36; ++i)
a.push_back (i);
// Here is another method to initilize your vector:
// std::vector<int> a {10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,
// 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35};
// Here you loop over all elements and only select the three first elements
// of every six elements:
for (int i=0; i<a.size(); ++i)
if (i%6 < 3)
std::cout << a[i] << std::endl;
return 0;
}
Оператор% дает остаток от деления двух значений типа int.