a stack of 3 identical books has a combined height of 27mm. If you take 2 of those books and add them to a stack of 5 books that are each 20mm thick, how high is the stack of 7 books
thickness of each book from first stack = 27/3 mm or 9 mm
2nd stack has 5 books, each 20 mm thick, so its height now is 100 mm
adding on 2 books of 9 mm thickness
====> 100 + 18 = 118 mm