The class leader plans to use the class fee to buy some diaries as prizes for recreational activities. If he buys 3.5 yuan diaries each, he will have 2.5 yuan left; if he buys the same number of 4.2 yuan diaries, he will be short of 2.4 yuan. So how many diaries does the monitor plan to buy?
班长计划用班费买一些日记本作为文娱活动的奖品，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．那么班长计划买多少本日记本．
<gadget id="calculator">2.4 + 2.5</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>