模拟

#include<iostream>
#include<string>
using namespace std;

int main(){
    int T;
    cin >> T;
    getchar();

    while(T --){
        string s1, s2;
        getline(cin, s1);
        int a = stoi(s1.substr(0, 2)) * 3600 + stoi(s1.substr(3, 2)) * 60 + stoi(s1.substr(6, 2));
        int b = stoi(s1.substr(9, 2)) * 3600 + stoi(s1.substr(12, 2)) * 60 + stoi(s1.substr(15, 2));
        if(s1.size() == 22){
            if(s1[20] == '1'){
                b += 24 * 3600;
            }
            else{
                b += 48 * 3600;
            }
        }
        getline(cin, s2);
        int c = stoi(s2.substr(0, 2)) * 3600 + stoi(s2.substr(3, 2)) * 60 + stoi(s2.substr(6, 2));
        int d = stoi(s2.substr(9, 2)) * 3600 + stoi(s2.substr(12, 2)) * 60 + stoi(s2.substr(15, 2));
        if(s2.size() == 22){
            if(s2[20] == '1'){
                d += 24 * 3600;
            }
            else{
                d += 48 * 3600;
            }
        }
        int t = (b - a + d - c) / 2;
        printf("%02d:%02d:%02d\n", t / 3600, t % 3600 / 60, t % 60);
    }
    return 0;
}