In recent years, new rules have been put in place to try to speed up the amount of time it takes to complete a professional baseball game. Before these rules were enacted, the mean length of a professional baseball game was about $172\text{ minutes}$.

Hank decided to do a statistical test to see if the mean length of a professional baseball game is now less than $172\text{ minutes}$. From a random sample of $20\text{ games}$ played under the new rules, he found the mean length of a game to be $163\text{ minutes}$ with a standard deviation of $21.2\text{ minutes}$.

When conducting his test, what value would Hank use for the standard error of the statistic?

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