This problem was solved by Ali and Prateek,
both from Riccarton High School in Christchurch, New Zealand, by
Malcolm and Hamish, both from Madras College in St Andrews,
Scotland, and by Daniella, Alice and Alice L from The Mount School
in York. Jacob from SSLC and Joshua from All Saints Junior School
also sent solutions.
Well done to you all - although we didn't
receive any solutions that explained exactly what you had done to
arrive at the answer.
Daniella, Alice L and Alice U sent this
Prateek sent us a different image of the
Ross, Brandon, Holly and Daniela from Ashford
Hill Primary wrote to say:
We have found that the numbers not in use are: $3$, $6$, $14$,