An empty tank was filled with water. Tap A was turned on at the start. Tap B was turned on 5 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 1 minute?
Tap B in 1 min = 30 − 10 = 20 litres
Ans: 20
How to solve it
Read the first 5 minutes, Tap A alone: the line rises from 0 to 50 litres, so Tap A adds 10 litres in 1 minute.
Read the next 5 minutes, both taps running: the line rises from 50 to 200 litres, which is 150 litres added.
Divide by 5 minutes: 150 divided by 5 is 30 litres in 1 minute, from the two taps together.
Tap A never stopped, so take its 10 litres off: 30 minus 10 is 20.
Tap B flowed 20 litres of water in 1 minute.
Tap A never stopped. Its ten litres is the bottom of this minute’s climb. Thirty minus ten is twenty. The orange twenty on top is Tap B. Twenty litres of water flowed from Tap B in one minute.
Practice questions with worked solutions
Question 1 is free to try. The other 8 open in the free trial, each with its full working.
Question 1 of 9 · Basic · free to tryTwo taps fill a tank. With both taps running, 21 L of water goes into the…Two taps fill a tank. With both…
Two taps fill a tank. With both taps running, 21 L of water goes into the tank each minute. The second tap on its own fills 13 L each minute. How much water does the first tap fill each minute?
How do you find each tap's rate in P5 rate graph and tap questions?
Here a rate is read from a line graph of volume against time, or from two taps filling or draining a tank.
What your child learns here
Here a rate is read from a line graph of volume against time, or from two taps filling or draining a tank. The page applies the Primary 5 Rate outcomes to combined rates, alongside finding the volume of liquid in a tank.
On a graph of volume against time, the rate for a straight stretch is the change in volume divided by the change in time.
A steeper stretch of the graph means a faster rate.
When two taps run together, their rates add.
One tap's rate is the combined rate minus the other tap's rate.
If the rate changes partway, the volume for each stretch is found separately and then added.
Does the answer make sense?
When two taps both fill, or two outlets both drain, their rates add, so the rate your child finds for one of them must be smaller than the rate with both running. On a line graph, the steeper stretch shows the faster flow, so its rate must come out bigger.
How to practise these questions
Basic questions train splitting a combined rate into the two taps' own rates and totalling the water over two stretches. Intermediate questions add a draining tank and a combined rate recovered from a volume and a time. Challenge questions read the rates off a line graph that bends where the change happens, or compare two graphs over one time axis. If the graph or the two stretches get mixed up, watch the solution film for that question, then try it again later from Revision.
Questions parents ask
How do you find a rate from a line graph? Take a straight stretch, find how much the volume rose and how many minutes it took, then divide the volume by the minutes.
Is this topic in the Primary 5 syllabus? It applies the Primary 5 Rate outcomes, and links with finding the volume of liquid in a rectangular tank from the Volume chapter. Reading a line graph comes from earlier levels.
What happens to the rate when a second tap opens? The rates add, so the water rises faster and the graph gets steeper. The second tap's own rate is the new rate minus the first tap's rate.
