(Use proportions to solve the problem) Stack up 12 books of the same thickness, and their height is 30 cm. If 20 such books are stacked, what is the new height?
(用比例解题)将12本相同厚度的书叠起来，它们的高度为30厘米．如果将20本这样的书叠起来，那么新的高度是多少？
<gadget id="calculator">30 / 12</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">(5/2) * 20</gadget>
<output>50</output>

<result>50</result>