If you choose to read 2 books of different subjects from 3 different Chinese books, 4 different mathematics books, and 5 different foreign language books, how many different choices are there?
如果从3本不同的语文书、4本不同的数学书、5本不同的外语书中选取2本不同学科的书阅读，那么共有多少种不同的选择．
<gadget id="calculator">4 * 3</gadget>
<output>12</output>

<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">12 + 15 + 20</gadget>
<output>47</output>

<result>47</result>