What does the symbol r indicate in an experimental design?

Apublic library wants to place 4 magazines and 9 books on each display shelf. the expressions 4s +9s represents the total number of items that will be displayed on s shelves. simplify this expression

Find the balance at the end of 4 years if 1000 is deposited at the rate of

Using two or more complete sentences, describe how you can find a vector parallel to b=-2,3