次の問題
以下のコードで`purchase`メソッドの返り値は何になりますか?
```ruby
stock_service = instance_double(StockService)
allow(stock_service).to receive(:fetch_stock_quantity).and_return(30)
purchase_manager = PurchaseManager.new(stock_service)
result = purchase_manager.purchase(123, 25)
```