Is the mean of the squares of two numbers greater than, or less
than, the square of their means?
Choose any two numbers. Call them a and b. Work out the arithmetic mean and the geometric mean. Which is bigger? Repeat for other pairs of numbers. What do you notice?
There are thirteen axes of rotational symmetry of a unit cube. Describe them all. What is the average length of the parts of the axes of symmetry which lie inside the cube?
Start by thinking about what these two things together tell you
"the mean of the first three was 9"
"the mean of all four was 15"