An arithmetic sequence is shown below. You are not given many terms of this sequence but you do have enough to fill in the missing number. What number goes in the blank 13 55 ...?

In an arithmetic sequence, the difference between consecutive terms is constant. To find the missing number, we can determine the common difference. The difference between the first term (13) and the second term (55) is 55 - 13 = 42. Thus, the missing term should be 13 + 42 = 55. Therefore, the missing number is 55.