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Four cousins — the mean age
The mean age of four cousins is 12 years. Three of the cousins are 9, 13, and 15 years old. Find the age of the fourth cousin.
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The pattern behind it: An average is a total
Turn the average back into a total before doing anything with it.
In this one. An average is the total shared equally, so multiply the mean by the number of cousins to recover the total, then subtract the known ages.
How to spot it
The word 'average' appears alongside a group being split into two parts, or two separate journeys or trips being combined into one overall figure.
Common mistake
Children average two averages directly, such as averaging two speeds or two group averages, without weighting by how many, or how long, each part actually covers.
Try one like it
Standard A bakery's week — average daily sales A bakery recorded its sales over 5 days: $42, $48, $55, $51 and $44. What was the average daily sales amount over the 5 days?The 33 problem patterns
All 33 patterns, one to a page — the 19 behind Primary 5 and 6 word problems, and the skills the younger years are built on. The bar model, how to spot it in the question, and the mistake children make. A booklet you can print. Free.
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