如图所示为水龙头 A、B 和一个空水箱。 下午 1:30，仅打开水龙头 A。水从水龙头 A 流入水箱，流速为每分钟 1.8 升。20 分钟后，打开水龙头 B。下午 2:00 时，水箱里装了一半的水。每分钟从水龙头 B 流出多少水？ 答：___ $\ell$

1. Calculate the total capacity of the tank in litres.

$\text{Capacity of tank} = 60 \times 25 \times 30 = 45000 \text{ cm}^3$
$45000 \text{ cm}^3 = 45 \text{ litres}$

2. Find the volume of water in the tank when it is half-filled.

$\text{Volume of water at 2 p.m.} = 45 \div 2 = 22.5 \text{ litres}$

3. Calculate the total volume of water that flowed in from tap A.

$\text{Time from 1.30 p.m. to 2.00 p.m.} = 30 \text{ minutes}$
$\text{Water from tap A} = 30 \times 1.8 = 54 \text{ litres}$

4. Calculate the volume of water that flowed out from tap B.

$\text{Water flowed out from tap B} = 54 - 22.5 = 31.5 \text{ litres}$

5. Calculate the rate of water flowing out from tap B.

$\text{Tap B was turned on 20 minutes after 1.30 p.m., so it was on for } 30 - 20 = 10 \text{ minutes.}$
$\text{Rate of tap B} = 31.5 \div 10 = 3.15 \text{ litres per minute}$