How can I make something happen x percentage?

I have to write a piece of code in the form of c*b, where c and b are random numbers and the product is smaller than INT_MAx. But b or c has to be equal to 0 10% of the time and I don't know how to do that.

    srand ( time(NULL) );
    int product = b*c;

    c = rand() % 10000;
    b = rand()  % INT_MAX/c;
            b*c < INT_MAX;

    cout<<""<<endl;
    cout << "What is " << c << "x" << b << "?"<<endl;
            cin >> guess;