01. Sum Seconds

#include <iostream>

using namespace std;

int main() {
    int firstTime, secondTime, thirdTime;
    cin >> firstTime >> secondTime >> thirdTime;

    int total = firstTime + secondTime + thirdTime;
    int minutes = total / 60;
    int seconds = total % 60;

    if (seconds < 10) {
        cout << minutes << ":0" << seconds << endl;
    }
    else {
        cout << minutes << ":" << seconds << endl;
    }
}

Решение с тернарен оператор:

#include <iostream>

using namespace std;

int main() {
    int firstTime, secondTime, thirdTime;
    cin >> firstTime >> secondTime >> thirdTime;

    int total = firstTime + secondTime + thirdTime;
    int minutes = total / 60;
    int seconds = total % 60;

    cout << minutes << ":" << (seconds < 10 ? "0" : "") << seconds << endl;

    return 0;
}

Решение със setW() и setfill() на библиотеката iomanip:

#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    int firstTime, secondTime, thirdTime;
    cin >> firstTime >> secondTime >> thirdTime;

    int total = firstTime + secondTime + thirdTime;
    int minutes = total / 60;
    int seconds = total % 60;

    cout << minutes << ':' << setw(2) << setfill('0') << seconds << endl;

    return 0;
}