I think that may have just been a typo. He/She probably meant to say what percent of the UK would be cut down per year which I think would be:
Which is already greater than 100%.
Unless that extra 3 was also a typo then it would be
In other words it would just be
"what would be the percentage of the rain forest cut down per year?"
we should not try second guessing the real question, but make clear that
the question asked makes no sense. That way the original poster may
learn something other than the answer to their question, and that is that
it is important to make their meaning clear.
It is what my supervisor taught me, and what his supervisor was well known