Means, and working backwards from a mean

The mean is the total shared equally. That makes it reversible: if you know the mean and how many things there are, you know the total, and a missing value is whatever makes the total come out right. Most contest questions use the mean backwards, not forwards.

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Work through it

LearnWhat it is, how to spot it, a worked example, and where it goes wrong.
Practice — 3-point ladderNo problems written at this tier yet.Nothing written at this tier yet
Practice — 4-point ladder2 problems. Three in a row finishes it.0/3 in a row · 0/2 solved
Practice — 5-point ladder2 problems. Three in a row finishes it.0/3 in a row · 0/2 solved
Skill quizFinish every ladder that has problems first.Finish the ladders above first

Before and after

Needs first

NUM-09Fractions of a quantity

Opens up

MOD-08Mixtures and weighted averages

In official papers

No indexed official paper question has been tagged with NUM-15 yet. The index grows one paper at a time, and it records only question numbers, point values and our own skill tags — never the problems themselves.

See the papers indexed so far

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