C 中的变量不会倍增

im在C 作业上工作。在这个项目中，我需要知道从出售股票收到的金额我通过将股票乘以销售_price来得到。但是，由于某种原因，当我运行代码时，我得到的结果仅是我输入的数字作为售价。为什么销售_price不乘以股票数？预先感谢。

//consoleapplication9.cpp：定义控制台应用程序的入口点。 //

#include "stdafx.h"
#include <iostream>
using namespace std;
int main()
{
    double stocks ;
    double purchase_price ;
    double selling_price ;
    cout << "Enter number of stocks sold:";
    cin >> stocks ;
    cout << "Enter purchase price:";
    cin >> purchase_price ;
    cout << "Enter selling price:";
    cin >> selling_price ;
    double invested = purchase_price * stocks + (purchase_price * 0.015);
    double total_service_charges = ((purchase_price + selling_price)* (.015 * stocks));
    cout << "Total money invested =" << invested << endl;
    cout << "total service charges=" << total_service_charges << endl;
    cout << "Amount received from selling stocks=" << (selling_price*stocks);
        if (purchase_price < selling_price)
        {cout << "Amount gained=" << selling_price - purchase_price;}
        else if (purchase_price > selling_price)
        {
            cout << "Amount lost=" << purchase_price - selling_price;
        }
        else if (purchase_price = selling_price)
        {
            cout << "Amount gained=" << purchase_price - selling_price<< endl;
        }

    return 0;
}