de_1.cpp

#include <bits/stdc++.h>
using namespace std;

int main() {
  int n, k, a[50], cnt = 0;
  cout << "Nhap n: "; cin >> n;
  for (int i = 0; i < n; i++) {
    cout << "Nhap a[" << i << "]: "; cin >> a[i];
    if (a[i] % 3 == 0) cnt++;
  }
  cout << "So so chia het cho 3: " << cnt << endl;
  cout << "Nhap k: "; cin >> k;
  cout << "Cac vi tri co gia tri bang k: ";
  for (int i = 0; i < n; i++) {
    if (a[i] == k) cout << i << ' ';
  }
}