Wang Gang has 20 red, blue and black pencils in total, among which the number of black pencils is 1 more than half of the number of red pencils, and the number of blue pencils is 1 more than half of the number of black pencils. How many blue pencils does Wang Gang have?
王刚有红、蓝、黑三种铅笔共20支，其中黑铅笔的支数比红铅笔的一半多1支，蓝铅笔的支数比黑铅笔的一半多1支．王刚有蓝铅笔多少支．
<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<gadget id="calculator">2 + 4 + 2 + 20</gadget>
<output>28</output>

<gadget id="calculator">1 + 2 + 4</gadget>
<output>7</output>

<gadget id="calculator">28 / 7</gadget>
<output>4</output>

<result>4</result>