How can n squares of side length 1 be arranged inside a larger square so that the latter is as small as possible? Clearly, if n = 4 or 9, we can simply take a larger square of side length 2 or 3. The question is far less straightforward when packing five squares. The best configuration is the one shown here, and the result was proved in 1979. Other results for small values of n were not proved until the early 2000s!
The case n = 11 is the smallest case that remains open.