There are 9 different mathematics reference books, 6 different Chinese reference books, and 3 different English reference books on a bookshelf. Now take out two reference books of different subjects, how many different methods are there?
在一个书架上有不同的数学参考书9本，不同的语文参考书6本，不同的英语参考书3本．现从中取出两本不同学科的参考书，有多少种不同的取法？
<gadget id="calculator">9 * 6</gadget>
<output>54</output>

<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">54 + 27 + 18</gadget>
<output>99</output>

<result>99</result>