First, let's take a look at the 3 by 3 sheet of stamps.
There are two vertical creases between the three columns of stamps.
These two creases comprise a total of six vertical stamp-lengths.
Similarly for the two horizontal creases, six horizontal stamp-lengths.
That makes a total of 12 stamp-lengths of creases.
It's given that there are 18 inches of crease in the 3 x 3 sheet.
Therefore these square stamps are 1.5 inches on a side.
Figure the 10 by 10 sheet the same way.
Nine vertical creases, creating 90 stamp-lengths of vertical crease.
Nine horizontal creases, creating 90 stamp-lengths of horizontal crease.
That totals 180 stamp-lengths of crease in the 10 x 10 sheet.
Since each stamp is 1.5 inches on a side, the total length of crease is 1.5 • 180 = 270 inches
.