Calculation of error in focal length? 
$u=-10cm$ $v=10cm$ 

Using the formula the focal length is 5 cm. But how do I get the fractional error in focal length when neither $\Delta u$ nor $\Delta v$ are specified?
The options given are 
A. (0.05±0.05) cm
B. (0.05±0.10) cm
C. (5.00±0.05) cm
D. (5.00±0.10) cm
 A: Error is $1 mm$ ,i.e. the least count as provided at the end of your axes where $1 cm $ is divided into 10 divisions.
$$f=\frac{uv}{u-v}$$
Fractional Error in $f$=fractional error in $uv$ + fractional error in $u-v$.
And fractional error in $uv$=fractional error in $u$+ fractional error in $v$.
Error in $u-v$= Error in $u$+error in $v$ // Note that it is not relative error
Can you continue from here?
A: This is a bit of a sloppy problem, because the numbers given ("10cm") are not in engineering notation.  Sans any other specifications, I would default to the uncertainty being 0.5cm , i.e. half the least significant digit provided.  Trouble is,  it could just as easily be a value rounded to the nearest ten-centimeter value,  implying the uncertainty is 5cm.   I do think Awesome's assumption of 1mm is highly optimistic.
If your teacher wrote this problem, ask him to be more careful about his notation.  If it's from a book,  just write down your assumptions about the uncertainty so you can justify your work.
