Is the mean of the squares of two numbers greater than, or less
than, the square of their means?
Can you use the diagram to prove the AM-GM inequality?
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"