Xiaoqiang wanted to buy a composition book with a unit price of 3.9 yuan. He added 0.7 yuan to his existing money to buy one copy. Later, Xiaoqiang only bought a copybook for 2.3 yuan. How much money does he have left?
小强想买一本单价为3.9元的作文书，他现有的钱再添0.7元正好买一本，后来小强只买了一本2.3元的字帖，他还剩多少元钱？
<gadget id="calculator">3.9 - 0.7 - 2.3</gadget>
<output>0.9</output>

<result>0.9</result>