HEB grocery wants to estimate the average transaction amount ($) for purchases. They know this is likely to vary by day of week, so they first look at transactions on Mondays. They know that the population standard deviation for their Monday transactions is $8.20.

From their computer records they found that for 2,860 transactions on previous Mondays, the sample average was $21.55. Purchase transaction amounts can be any fractional dollar value.

Find a 85% confidence interval for the average transaction amount on Mondays.

$21.39 to $22.71

$9.74 to $33.36

$21.25 to $21.85

$21.30 to $21.80

$21.33 to $21.77