Two people, A and B, start from a certain point on the ring road at the same time, and walk with their backs. It is known that it takes 90 minutes for A to cycle around, and they meet each other 30 minutes after starting. Ask: How long does it take for B to cycle around.
甲、乙二人骑车同时从环形公路的某点出发，背向而行，已知甲骑一圈需90分钟，出发后30分钟两人相遇，问：乙骑一圈需多少分钟．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / 90</gadget>
<output>1/90 = around 0.011111</output>

<gadget id="calculator">(1/30) - (1/90)</gadget>
<output>1/45 = around 0.022222</output>

<gadget id="calculator">1 / (1/45)</gadget>
<output>45</output>

<result>45</result>