Reflection and Mirrors - Mission RM7 Detailed Help

In a classroom demonstration, a physics teacher places a candle a distance of 68.1 cm from a concave mirror and measures an image distance of 94.8 cm. What is the focal length of the mirror? 


 
The mirror equation relates the object distance (do), the image distance (di) and the focal length (f):
 
1 / do + 1 / di = 1 / f


 
The mirror equation, with its three reciprocal terms, is a little awkward to work with. The focal length (the unknown) is already by itself on the right side of the equation; so an algebra step is not needed here. Simply substitute the two numerical values into the equation and use your calculator to evaluate the left side. Once done, the reciprocal of the focal length is equal to the number on your calculator. Take the reciprocal of this number to determine the focal length.


 
The Minds On Physics program calculates answers accurate to the fourth decimal place. Your answer does not need to be that accurate. The program will allow you to have a 1 percent deviation from the right answer without being wrong. This means two things: 1) Enter your answer with at least three significant digits. 2) Do not round any numbers until your final calculation.