There are 144 white rabbits and black rabbits on the grass. At this time, there are 6 white rabbits. Now the number of white rabbits on the grass is exactly 4 times that of black rabbits. How many black rabbits are there?
草地上白兔、黑兔共144只，这时又来6只白兔，现在草地上的白兔只数正好是黑兔的4倍，黑兔有多少只？
<gadget id="calculator">144 + 6</gadget>
<output>150</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">150 / 5</gadget>
<output>30</output>

<result>30</result>