Please help I don't know what it is

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

45% of the trees in a park are apple trees. there are 27 apple trees in the park. how many trees are in the park in all?

If you have 12 feet of string and you cut it into equal length of 5 inches each how much string will be left