It's not about the 85% that matters.
For maths, and for physics, the most important is your
presentation of your answers. You may get the final answer correct,
but if bad presentation takes away a lot of your marks, you will
still not be able to score.
So while you prepare for your O levels, take careful note on
your presentation too.
I always tell my students, help the markers give you marks by
being clear in your presentations.