Question 864870
Let x = score on the final (the decimal form of the percentage, so for example if we got x = 0.6, then that's a 60% grade on the final)



(75% of current work) + (25% of exam) = final overall grade



75% of 78% + 25% of x = 85%



0.75*0.78 + 0.25x = 0.85



0.585 + 0.25x = 0.85



0.25x = 0.85 - 0.585



0.25x = 0.265



x = 0.265/0.25



x = 1.06



So you need to get 100*1.06 = 106% on the final. This means that without extra credit, it is not possible to go from a 78% to an 85%. The best you can get is 0.75*0.78 + 0.25*1 = 0.835 which is 83.5%



Note: again there may be extra credit of some form and the teacher may scale things differently, curve the grade in some fashion, or drop your lowest score. The teacher may do any of these things or a combo of them or s/he may do something else entirely to alter the grade. So this is why it's best to ask the teacher directly. However, with this math, you get a basic idea of where you stand and where you could end up.