The capacity of each small box of milk is 0.25 liters, and the capacity of each large box of milk is 0.5 liters. Xiaohong bought 18 boxes of milk in the large box and 18 boxes in the small box. How many liters did she buy in total?
每小盒牛奶的容量是0.25升，每大盒牛奶的容量是0.5升．小红买了大盒和小盒牛奶各18盒，一共买了多少升？
<gadget id="calculator">0.25 + 0.5</gadget>
<output>0.75</output>

<gadget id="calculator">0.75 * 18</gadget>
<output>13.5</output>

<result>13.5</result>