Explanation
Now magnification is given by:
M=\frac{-v}{u}
And mirror formula
\frac{1}{v}+\frac{1}{u}=\frac{1}{f}
Multiply by u
\frac{u}{v}+\frac{u}{u}=\frac{u}{f}
\frac{u}{v}+1=\frac{u}{f}------------(1)
We know M=\frac{-v}{u}
\frac{1}{M}=\frac{-u}{v}
\frac{u}{v}=\frac{-1}{M}
Putting in (1)
\frac{-1}{M}+1=\frac{u}{f}
\frac{-1}{M}=\frac{u-f}{f}
So
M=\frac{f}{f-u}
Now u=-20
M= \frac{f}{f-(-20)} and real image means M is negative
-3= \frac{f}{f+20}
-3f-60=f
f=-15 cm
M= \frac{-15}{-15+30}
M= -1
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