s = listBox1->Items[i];

for (int i = 0; i < listBox1->Items->Count; i++) {
            string s;
            int a, b;
            s = listBox1->Items[i];
            a = stoi(s.substr(0, s.find('/')));
            b = stoi(s.erase(0, s.find('/') + 1));
            label1->Text += a/b;
        }

String^ s = listBox1->Items[i]->ToString();
array<String^>^ splitted = s->Split('/');
int a = Int32::Parse(splitted[0]);
int b = Int32::Parse(splitted[1]);
label1->Text += (a/b).ToString();

auto ms = listBox1->Items[i]->ToString();

#include <msclrmarshal_cppstd.h>
...
s = msclr::interop::marshal_as<std::string>(ms);