Question

Two lenses of focal lengths 20cm and -40cm are kept in contact with each  other. The power of equivalent lens will be

Answer 1

20cm=0.2m
-40cm=-0.4m
power of 1st lens=1/0.2=0.5
power of 2nd lens=-0.25
total power=0.5 + (-0.25)
=0.25