On Children's Day, Grandpa Liu took his three grandsons and one granddaughter to the park to play. Children's tickets are half priced at 7.5 yuan each. How much did they spend on the tickets?
儿童节那天，刘爷爷带着3个孙子和一个孙女去公园玩，儿童门票半价为每张7.5元，他们买门票一共要花多少钱？
<gadget id="calculator">7.5 * 4</gadget>
<output>30</output>

<gadget id="calculator">7.5 * 2</gadget>
<output>15</output>

<gadget id="calculator">30 + 15</gadget>
<output>45</output>

<result>45</result>