The amount of milk in the large box is 2.5 times that of the small box, and the price of the small box is 2.5 yuan. The price of the small box is 1.40 yuan cheaper than that of the large box. Please come up with a mathematical problem and solve it with parallel equations.
大盒的牛奶量是小盒的2.5倍，小盒2.5元，小盒的价格比大盒便宜1.40元．请你提出一个数学问题并列方程解答．
<gadget id="calculator">2.5 + 1.4</gadget>
<output>3.9</output>

<result>3.9</result>