The class plans to buy some diaries as prizes. If you buy diaries of 3.5 yuan each, you will have 2.5 yuan left; if you buy the same number of diaries at 4.2 yuan each, you will be short of 2.4 yuan. So how many diaries does the class plan to buy?
班级计划买一些日记本作为奖品，如果买每本3.5元的日记本，将剩余2.5元；如果买每本4.2元的同样数量的日记本，将缺少2.4元．那么班级计划买多少本日记本？
<gadget id="calculator">2.5 + 2.4</gadget>
<output>4.9</output>

<gadget id="calculator">4.2 - 3.5</gadget>
<output>0.7</output>

<gadget id="calculator">4.9 / 0.7</gadget>
<output>7</output>

<result>7</result>