Three lenses having powers 2 D, 2.5 D and 1.7 D are kept touching in a row. What is the total power of the lens combination ?

Three lenses having power 2, 2.5 and 1.7 D are kept touching in a row. What is the total power of the lens combination?

Three lenses having power 2, 2.5 and 1.7 D are kept touching in a row. What is the total power of the lens combination?

Two lenses of power +1.5D and -0.5D are kept in contact on their principal axis . What is the effective power of the combination ?