>>148
(2)
#include<iostream>

int main(void){
int N=0;
double value, average=0.0;

std::cout << "Input N : ";
std::cin >> N;

for(int i=0;i<N;i++){
std::cout << "Input data[" << i << "] : ";
if(!(std::cin >> value)){
std::cerr << "\nERROR : Unknown input.\n";
return 1;
}
average=(average*i+value)/(i+1);
}
std::cout << "N=" << N << " average=" << average << std::endl;

return 0;
}