This series comprises four traditional caches, each of which
contains a ten digit number. Any three of these numbers provide
enough information to find the bonus cache - which three will you
choose? The numbers are labelled A, B, C and
D, and sum to 10,000,000,000: the tenth root of a googol.
The grid reference of the final cache (in grid square SP) can be
calculated as A – B + C – D.
A quincunx is an arrangement of five dots as seen on the
fifth side of a playing die, with one at each corner of a square,
and one in the centre. It is named after the Roman coin worth five
twelfths (quinque + uncia) of an as,
emblazoned with that pattern.
This cache is right beside an important Oxford thoroughfare. The
container is a magnetised Tic-Tac box, and is probably not
watertight, so please replace it upside-down so that water doesn't
get in. The number for the bonus cache is on the side. You will
need to bring your own writing implement.