d = 20 cm
u + v = 90 cm
v – u = 20 cm
v = 55 cm
u = 35 cm
Lens formula:
1v−1u=1f\frac{1}{v} - \frac{1}{u} = \frac{1}{f}v1−u1=f1
155−1−35=1f\frac{1}{55} - \frac{1}{-35} = \frac{1}{f}551−−351=f1
155+135=1f\frac{1}{55} + \frac{1}{35} = \frac{1}{f}551+351=f1
35+551925=1f\frac{35 + 55}{1925} = \frac{1}{f}192535+55=f1
f = 21.38 cm
Answer: 21.38 cm
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