Given an arithmetic progression, the first term is 11, the last term is 101, and the sum is 504. How many terms are there in this series?
已知一个等差数列，首项是11，末项是101，总和是504，这个数列共有多少项？
<gadget id="calculator">11 + 101</gadget>
<output>112</output>

<gadget id="calculator">2 / 112</gadget>
<output>1/56 = around 0.017857</output>

<gadget id="calculator">504 * (1/56)</gadget>
<output>9</output>

<result>9</result>