1 When this strategy is compared with the alternative strategy of keeping the gold in the portfolio for one year we see that investor is better off by 15 dollars per ounce.
这里的off 是payoff 意思吗？

２Assuming that individuals are always willing to take advantage of arbitrage opportunities when they arise,we can concludes that the
activities of traders should cause the one-year forward price of gold to be exactly 315 dollars.
这里的arise作上涨的解释吗？在金融里都知道需求的关系只能接近理论，为什么这里用exactly，难道exactly也有近似的意思吗？

1. better off: In a more favorable position or financial circumstances.

For example, They were better off flying than driving there, or They were better off than most of their neighbors.

2. (1) arise: To come into being; originate: eg. hoped that a new spirit of freedom was arising.

Here, "when they arise" means 'when opportunities come into existence...".

(2) Here, EXACTLY means Just as it should be.