At the time of delivery, the spot price and futures price tend to be consistent, which means that the price difference between 40 yuan and 50 yuan is 10 yuan no matter how the market outlook goes.

100 shares is100x10 =1000 yuan.

Interest is usually calculated at compound interest.

The final profit is the money earned by arbitrage minus interest 1000-840= 160.

I think this is easier to understand.