Forget about chances, just do loads of questions until you have a gigantic pile of papers and books.
I got a B in GCSE Maths (definitely not natural gifted and I have no talent in Maths), and though I hated Maths I decided to do both Maths and Further Maths. At first my grades were below average and kept getting U's, yet I put huge time in practicing and done every single past paper twiceand even started doing other exam board past papers. Days before the exam I've managed to achieve above 90% easily.
So if you want an A* then start working harder than everyone else early.