The school science and technology team collects rock specimens. 20 pieces of granite were collected, which were limestone (4/5), and the marbles collected were granite (3/4). How many pieces of marble rock have been collected?
学校科技小组采集岩石标本．采集到花岗岩20块，是石灰岩的(4/5)，采集到的大理石是花岗岩的(3/4)．采集到大理石岩多少块？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">20 * (3/4)</gadget>
<output>15</output>

<result>15</result>