钱的价值量和社会产品的价值量是相等的

First of all, quantity theory of money should be written as MV = PY , where V is the number of times in a year that a dollar is used to purchase outputs. It means the total value of money in circulation equals to the total value of outputs. The k you have there is actually 1/v, it is a constant in the shot run.

谢谢大家，后来我问了助教，她说这里的p不是具体一件商品多少钱，而是一个加权平均的价格指数。
m是名义货币量，m/p是实际货币量。
书上的例子我觉得不好，太具体了：一个只生产面包的经济，货币量是10美元，面包单价0.5美元，那么货币存量能买20个面包。
二楼的解释也有道理。
三楼的不会是国际友人吧。