foo.cpp: In function 'int main()':
foo.cpp:30:15: error: 'scanf' was not declared in this scope
  scanf("%f",&s);
               ^
foo.cpp:46:32: error: 'printf' was not declared in this scope
  if(ans>s)printf("impossible/n");
                                ^
foo.cpp:47:44: error: 'printf' was not declared in this scope
  else printf("Need %.2f miles of cable",ans);
                                            ^