Xiao Zhang bought several exercise books at the price of 1 yuan to buy 3 exercise books. These exercise books were sold at the price of 2 yuan and 5 exercise books. How many exercise books must he sell to make a profit of 10 yuan?
小张以1元钱买3个练习本的价格买进练习本若干个．又以2元5个练习本的价格将这些练习本卖出．如果要赚得10元钱的利润，他必须卖出多少个练习本？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(2/5) - (1/3)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">10 / (1/15)</gadget>
<output>150</output>

<result>150</result>