Xiaoling went to the park with grandpa, grandma, father, and mother to play, and saw the ticket price written at the entrance: 4.5 yuan per ticket for adults and 2.5 yuan per ticket for children. Please do the math for her, how much does it cost to buy tickets?
小玲和爷爷、奶奶、爸爸、妈妈一起去公园游玩，只见入门处写着门票价格：成人每张4.5元，儿童每张2.5元．请你帮她算一算，买门票一共要多少元钱？
<gadget id="calculator">4 * 4.5</gadget>
<output>18</output>

<gadget id="calculator">18 + 2.5</gadget>
<output>20.5</output>

<result>20.5</result>