A professor has recorded exam grades for 20 students in his class, but one of the grades is no longer readable. If the mean score on the exam was 81 and the mean of the 19 readable scores is 85, what is the value of the unreadable score?

20 students....mean score of exam was 81...

x / 20 = 81
x = 81 * 20
x = 1620....this is the total of all the grades added up

(1620 - x) / 19 = 85
1620 - x = 85 * 19
1620 - x = 1615
-x = 1615 - 1620
-x = - 5
x = 5 <=== weird answer...but thats what I am getting