Lele's father gave him 25 yuan to buy books and 2.4 yuan to buy 6 exercise books. When buying books, he found that he didn't have enough money to buy books, so he had to take out part of the money for buying exercise books. In this way, he just bought 4 exercise books. How much did he spend on the book this time?
乐乐的爸爸给他25元去买书，2.4元去买6本练习本．买书时他发现买书的钱不够，从买练习本的钱中拿出一部分后才够．这样，他正好买了4本练习本．这次他买书花了多少钱？
<gadget id="calculator">2.4 / 6</gadget>
<output>0.4</output>

<gadget id="calculator">0.4 * 4</gadget>
<output>1.6</output>

<gadget id="calculator">25 - 1.6</gadget>
<output>23.4</output>

<result>23.4</result>