Question 815627
The final exam counts for 40% of your grade (given). So the remainder of the class assignments (hw, quizzes, etc) is 100% - 40% = 60% of your grade. 


Let x = grade on the final exam


(weight of exam)*(grade on exam) + (weight of remainder)*(grade of remainder) = desired grade


(0.40)*(x) + (0.60)*(0.85) = 0.70 ... Note: percentages are converted to decimal form


(0.40)*(x) + (0.60)*(0.85) = 0.70


0.40x + 0.51 = 0.70


0.40x = 0.70 - 0.51


0.40x = 0.19


x = 0.19/0.40


x = 0.475


So this means you need to get a <font color="red">47.5%</font> or higher on the final exam to get a 70% or higher for an overall grade.


Note: the 85% is clearly above 70% (quite significantly too), which explains why you're given so much slack and you can get such a low score on the final (and still get away with a decent grade).


Another Note: your professor may grade on a curve or have other rules for grading (such as dropping the lowest score). It's best to ask the professor directly about questions like this. However, this gives you a basic idea of what you should get.