There are two cylindrical containers A and B. Initially, A contains 2 liters of water, and B is empty. Now inject a flow rate of 0.4 liters per minute into the two containers. After 4 minutes, the water levels of the two containers will be equal. Suppose the radius of the bottom of B is 5 cm. What is the radius of the bottom of A?
有A、B两个圆柱形的容器，最初在A里装有2升的水，B是空的．现在往两个容器里以每分钟0.4升的流量注入，4分钟后，两个容器的水面高度相等，设B的底面半径是5厘米，求A的底面半径是多少？
<result>7.5</result>