Explanation
Now magnification is given by:
M=−vu
And mirror formula
1v+1u=1f
Multiply by u
uv+uu=uf
uv+1=uf------------(1)
We know M=−vu
1M=−uv
uv=−1M
Putting in (1)
−1M+1=uf
−1M=u−ff
So
M=ff−u
Now u=−20
M=ff−(−20) and real image means M is negative
−3=ff+20
−3f−60=f
f=−15cm
M=−15−15+30
M=−1
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