The 90 moves to the right when you click off of that box
If you look at the top of the grade book you'll se that
C6 is listed on the top left. Look at the cell or box where
you entered 90 and notice that it is in column C, row 6.
You will see 90 written next to the equals (=) sign at the
top of the grade book. Everything you type into a cell goes
in this box as well.