You are given lenses with powers + 10D,+ 5D, - 5D, -20D,and- 10D.Taking a pair of lenses at a time, which two lenses will you select to have a combination of total focal length when the two lenses are kept in contact in each case.
(i) 20 cm (ii) - 10 cm (iii) - 20 cm (iv) 20/3 cm
