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f = 15

$m=-\frac{v}{u}=+\frac{1}{2}$            

  $v=-\frac{u}{2}$           

$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$            

$\frac{2}{-u}+\frac{1}{u}=\frac{1}{f}$            

u = – f = – 15cm

$\frac{1}{20}=\left(1.5-1\right)\left(\frac{2}{R}\right)$

R = 20cm

$\frac{1}{f\text{'}}=\left(\frac{1.5}{1.6}-1\right)\left(\frac{2}{R}\right)$            

$=\frac{-1}{16}\times \frac{2}{20}$                  

f' = – 160cm