Say if your exam is out of 400 UMS. You need 360 for an A*- 90% UMS
Each exam is worth 25%. So three exams in one science would make 75% up.
25% of the qualification is coursework. So one coursework is equivalent to an exam.
Say if each exam and coursework is worth 100 UMS.
If you got a low B- so 70 UMS- on the borderline.
Therefore you are at 370 UMS already by losing 30 from the 300.
So , you need to get very high A* on every single exam for one science in order to secure a high UMS needed to compensate for the lower score in the coursework- over 96 in fact!
Definitely difficult to achieve but nevertheless possible.
Hope I helped.