Question 879381
It depends upon how the scores are weighted.  How much is the test worth relative to what you've already completed?  I'll show you how to do it, and if you need help figuring it out, post again with more information.
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Let's assume that 67.9 is your average for 6 exams thus far.
That would be 405.6 points out of a possible 600.
Since your new exam is REPLACING the lowest score, figure out the difference in the scores (70-45) and add that to your total.
In this case, you would now have 405.6 +25, or 430.6.  That's a 71.8% instead of the 67.9%.  Could definitely make the difference in a letter grade.