The distance between A and B is 6 km. A truck travels from point A to point B at a speed of 30 meters per minute. When the truck leaves place A for 900m, a student catches the truck with a ball on a motorcycle at a speed of 90m per minute. After catching up, put the ball on the truck and return immediately. After returning, he chases the truck with a ball. Going back and forth like this, how many minutes after the truck departed for the last time to catch up with the truck.
A、B两地相距6千米．一辆货车以每分钟30m的速度由A地开往B地．货车离开A地900m时，一同学在A地乘摩托车带一个球以每分钟90m的速度追货车，追上后将球放在货车上立即返回，返回后再带一个球追货车．如此往返，最后一次追上货车是在货车出发后多少分钟．
<gadget id="calculator">120 + 60</gadget>
<output>180</output>

<result>180</result>