The volume of water in a tank is in the Primary 5 Volume chapter. The lesson film is the one Mathweave teaches this skill with in Primary 6, and the practice set below is chosen for Primary 5.
Lesson video and notes
Finding Water Level from Volume and Base Area
2 min 49 s
This lesson is shared with Primary 6.
How to solve it
Pouring does not change the water. A wider floor means a lower level.
Find Tank A's base area: 25 times 16 is 400 square centimetres.
Find the water's volume using its depth of 12 centimetres, not the tank's height: 400 times 12 is four thousand, eight hundred.
Find Tank B's base area: 40 times 15 is 600 square centimetres.
Divide the volume by Tank B's base area: four thousand, eight hundred divided by 600 is 8 centimetres deep.
Same water. A wider floor. A lower level.
Practice questions with worked solutions
Question 1 is free to try. The other 8 open in the free trial, each with its full working.
The depth of water in a rectangular tank is its volume divided by the tank's base area.
What your child learns here
The depth of water in a rectangular tank is its volume divided by the tank's base area. Your child uses that to find the level after water is poured in, drawn off, or moved into a tank with a different base. It is the Primary 5 rule for the volume of liquid in a tank, worked backwards, with litres changed into cm³ along the way.
The volume of water is length × breadth × the depth of the water.
Divide the volume of water by the base area to find its depth.
Change litres to cm³ before dividing: 1 litre is 1000 cm³.
Water moved to another tank keeps its volume, so only the base area changes.
If the second tank already holds water, the new depth is the old depth plus the rise.
Does the answer make sense?
In Question 1, the base is 60 cm by 40 cm, which is 2400 cm², and 9600 cm³ divided by 2400 cm² is 4 cm. Multiply back: 2400 times 4 is 9600. The depth must also be less than the tank's 15 cm height, and 4 cm is.
How to practise these questions
Question 1 is free: a tank 60 cm by 40 cm by 15 cm holds 9600 cm³ of water, and your child finds its depth. It is the one Basic question. The Intermediate questions draw off water given in litres, fill a tank to a fraction of its height, pour water into an empty tank with a different base, pour into a tank that already holds some water, and combine water from two containers in a tank with a square base. The two Challenge questions find the edge of a cubical tank from the water needed to fill its last 8 cm, and pass water through three tanks whose base areas shrink each time. Ask your child to write the base area of every tank on its own line first, since each depth is found by dividing by it.
Questions parents ask
Is the water level in the Primary 5 syllabus? The volume of liquid in a rectangular tank is a Primary 5 outcome, and finding a height from a volume and a base area is listed at Primary 6. This page uses the Primary 5 rule in reverse, so your child meets the Primary 6 idea with familiar numbers.
Why does the water stand lower in a wider tank? The same volume is spread over a larger base, so each centimetre of depth holds more water. Fewer centimetres are needed to hold it all.
What if the tank already has water in it? Find how far the poured water raises the level by dividing its volume by the base area, then add that rise to the depth already there.
