Sequence 1, 3, 5, 7... What is the 20th term of this arithmetic sequence.
数列1，3，5，7…这个等差数列的第20项是多少．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">19 * 2</gadget>
<output>38</output>

<gadget id="calculator">1 + 38</gadget>
<output>39</output>

<result>39</result>