A grocery store finds that the number of boxes of a new serial sold increases each week. In the first week, only 20 boxes of the serial were sold. In the second week, 53 boxes of cereal or sold and then the third week 86 boxes of cereal or sold. The number of boxes of cereal so each week represents an arithmetic sequence. What is the explicit rules for the arithmetic sequence that defines the number of boxes of cereal sold in week n? Use this rule to calculate how many boxes of cereal will be sold during the seventh week show all work