Simplify the following text for a Grade 8 native English-speaking person.
Original: In general , mathematical calculations are performed by aligning a mark on the sliding central strip with a mark on one of the fixed strips , and then observing the relative positions of other marks on the strips .
Simplified: Mathematical calculations are done by aligning a mark on the sliding central strip with one on one of the fixes strips . The relative position of other marks can then be observed .