up in a table of the standard normal distribution to find that the probability
of getting a z of less than -1 is 0.1587.
That both trees are under 20cm has probability 0.1587^2 ~= 0.025
That one is under and the other over 20cm means either the first is under
and the second over or the first is over and the seond under so the required
probability is 0.1587*(1-0.1587) + (1-0.1587)*0.1587 ~= 0.267