Question

 A person is given lenses of power +5D,-10D, +10D , -20 D considering a pair of lenses at a time, which two lenses he will select to get focal length of combination equal to -10cm.

10D, -20 D

-10 D, +10 D

+5 D, -10 D

-20 D, +5 D

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Answer 1

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Answer 2

$✭Answer ✭$

Toatal power, P= =P1+P2 and total length f is 1/5 =11/f+21/f

When lenses of 10 D and  -5 D are taken , total power 

P= =P1+P2 

= 10 D-5 D = 5 D

Hence, focal length = 100/5=20cm