с использованием if и switch.double uah;int answer;const double eur = 28.05;const double usd = 26.74;const double rub = 0.33;cout << "-------Menu-------" << endl;cout << "1 - EUR" << endl;cout << "2 - USD" << endl;cout << "3 - RUB" << endl;cout << "0 - Exit" << endl;cin >> answer;switch (answer){case 1:cout << "Enter UAH:";cin >> uah;cout << uah / eur << " EUR" << endl;break;case 2:cout << "Enter UAH:";cin >> uah;cout << uah / usd << " USD" << endl;break;case 3:cout << "Enter UAH:";cin >> uah;cout << uah / rub << " RUB" << endl;break;case 0:exit(0);break;default:cout << "Error!" << endl;break;}system("pause");