Stage: 4 Challenge Level: 
There are four unknown numbers.
The mean of the first two numbers is 4, and the mean of the first three numbers is 9.
The mean of all four numbers is 15.
If one of the four numbers is 2, what are the others?
physics. Modulus arithmetic. Rigid Bodies. Mean. Groups. Inverses. Short problems. Forces. engineering. Maths in STEM.
Published June 2006.
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