Finding the Average

1. 1Pour Until LevelThe fuller tanks pour into the emptier ones, pour after pour, until all three surfaces line up.Fuller tanks pour into emptier ones
2. 2One Level for AllEvery tank now holds exactly 10 litres. The dashed line touches all three surfaces — that shared level is the average.Average level: 10 litres
3. 3The Formula Behind the PouringPouring only moved water, so the total is still 6 + 9 + 15 = 30 litres. Sharing that total among 3 tanks gives 30 ÷ 3 = 10 litres each.(6 + 9 + 15) ÷ 3 = 10
4. 4One Bar, Shared EquallyThe three tanks slide together and merge into one bar of 30 litres. Slice it into 3 equal parts and each part is exactly the level we poured to: 10 litres.30 ÷ 3 = 10
5. 5Check the AnswerCheck: 3 tanks of 10 litres make 10 × 3 = 30 litres, and 6 + 9 + 15 = 30 litres. The two totals agree, so 10 litres is right.10 × 3 = 30 ✓
6. 6The RuleThe average of 6, 9 and 15 is 10 litres — not the middle tank (9 litres). Add everything, then share the total equally.Average = total ÷ count

Change the numbers to test the engine with any sum & difference problem: