There are two parallelograms A and B, their heights are equal, and the corresponding bases are a cm and 3a cm respectively. The area of A is 15 square centimeters, and the area of B is how many square centimeters.
有甲、乙两个平行四边形，它们的高相等，对应的底边分别是a厘米、3a厘米，甲的面积是15平方厘米，乙的面积是多少平方厘米．
<gadget id="calculator">3 * 15</gadget>
<output>45</output>

<result>45</result>