Mr. Wang bought chairs for 45 yuan each. He paid 900 yuan and got 90 yuan back. How many chairs did Mr. Wang buy back?
王老师买椅子，每把椅子45元，他付了900元，找回90元，王老师买回几把椅子？
<gadget id="calculator">900 - 90</gadget>
<output>810</output>

<gadget id="calculator">810 / 45</gadget>
<output>18</output>

<result>18</result>