This maths question for 10-year-olds has grown ups stumped

One dad was scratching his head at a maths
problem his son got at school. The puzzle, aimed
at Year 5 students in Derbyshire, UK, has drawn a
blank from even pro problem solvers.
The dad, 43, who got A-levels in maths in
economics, said: “My son’s grandma had spent a
while helping him with his homework and most of
it was straightforward but this one question left
her stumped. I then spent an hour or so trying to
work it out but found it impossible. I even sent it to
a friend who got a 1st class degree in economics
and they were baffled by it as well. A couple of
friends are maths teachers too so I’ve sent it to
them but so far I’ve heard nothing back. I really do
think it is impossible and it is certainly not
something a ten-year-old can answer. It just does
not make sense.” Do have a crack at it after the
cut...