Master Zhang bought several apples at a price of 4 apples for 1 yuan, and sold these apples at a price of 5 apples for 2 yuan. If he wants to make a profit of 15 yuan, how many apples must he sell?
张师傅以1元4个苹果的价格买进苹果若干个，又以2元5个苹果的价格把这些苹果卖出，如果他要赚得15元的利润，那么他必须卖出苹果多少个．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(2/5) - (1/4)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">15 / (3/20)</gadget>
<output>100</output>

<result>100</result>