A, B, and C start from A to B at the same time, C walks first, A rides a bicycle to take B to D on the way, B walks down to B, A meets C on the way back, and takes C to B As a result, three people arrive at B at the same time. It is known that they walk 4 kilometers per hour on foot and 12 kilometers per hour on bicycle. The distance between A and B is 90 kilometers. How many kilometers did B walk? (Solution of column equation)
甲、乙、丙三人同时从A地出发去B地，丙先步行，甲骑自行车带乙到途中D处，乙下边向B地步行，甲骑自行车返回中遇到丙带丙再去B地，结果三人同时到达B地，已知步行每小时走4公里，骑自行车每小时走12公里，A、B两地距离为90公里，求乙步行了多少公里？(列方程解)
<gadget id="calculator">90 / 4</gadget>
<output>45/2 = around 22.5</output>

<gadget id="calculator">90 / 12</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">(45/2) - (15/2)</gadget>
<output>15</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">1.5 / 12</gadget>
<output>0.125</output>

<gadget id="calculator">(1/8) - 0.125</gadget>
<output>0</output>

<gadget id="calculator">(3/4) + 0</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">15 / (3/4)</gadget>
<output>20</output>

<gadget id="calculator">3 * 20</gadget>
<output>60</output>

<gadget id="calculator">90 - 60</gadget>
<output>30</output>

<result>30</result>