There is an arithmetic sequence: 3, 7, 11, 15... What is the 100th item of this arithmetic sequence.
有一等差数列：3、7、11、15…这个等差数列的第100项是多少．
<gadget id="calculator">100 - 1</gadget>
<output>99</output>

<gadget id="calculator">99 * 4</gadget>
<output>396</output>

<gadget id="calculator">3 + 396</gadget>
<output>399</output>

<result>399</result>