Because the topic is about buying two products, and which installment method to buy is independent of each other. That is to say, the first item buys 2, and the second item buys 4. It is different from the first purchase 4 and the second purchase 2. This can be explained by binomial distribution. If you don't understand what I said above, please tell me and I will explain it to you with binomial distribution.
Hope to adopt, thank you (. #)