How it works
The same screen as the twenty game, with bigger numbers: a sum, a number pad, and no
clock. The help waits in the same way, for the third miss, and with bigger numbers it
says to work out the tens first and then the units, because that is the structure the
answer depends on.
A wrong answer never passes and the question never disappears. Nothing is lost by
being wrong except the difference between ten points and four.