Rate off a graph

An empty tank was filled with water. Tap A was turned on at the start. Tap B was turned on five minutes later and the two taps then ran together until the tank was full. The graph shows the volume of water in the tank. How many litres of water flowed from Tap B in one minute? Try it first. Pause if you like. Watch the pouring behind this graph. The tank and graph share the same litre scale. Tap A opens. The water rises, and the dot rises beside it at the same height. After five minutes, the tank holds fifty litres. At five minutes, Tap B opens as well. Tap A stays open. Both taps are pouring. The water rises faster, and the line is steeper. At ten minutes, the tank is full and both taps are turned off. No water goes in. The level stays the same, so the line runs flat. Read the first five minutes, when Tap A was running on its own. It starts at zero litres. It reaches fifty litres. Fifty minus zero is fifty litres added in the first five minutes. Fifty divided by five is ten litres in one minute. On this stretch, move one minute across. The line rises ten litres in that minute. Now the next five minutes, with both taps running. Start from fifty litres. End at two hundred litres. Two hundred minus fifty is one hundred and fifty litres added. One hundred and fifty divided by five is thirty litres in one minute. Move one minute across on this stretch. The line rises thirty litres in that minute. Thirty litres in one minute is what the two taps do together. Tap A never stopped. Its ten litres is the bottom of this minute’s climb. Thirty minus ten is twenty. The pink twenty on top is Tap B. To find a tap's rate, read one minute of each stretch, then subtract the other tap from both taps together.