Teacher Wang brought 800 yuan, 638 yuan was used to buy a learning machine, and 72 yuan was used to buy a calculator. How much money is left?
王老师带了800元钱，买一台学习机用去了638元，买一台计算器用去72元，还剩多少元？
<gadget id="calculator">800 - 638 - 72</gadget>
<output>90</output>

<result>90</result>