Answer:
a).

b). V = 15 units
Explanation:
Focal length F of a lens made by combining two lenses of different focal lengths U and V will be,

A). By solving the given formula,



B). If F = 6 and U = 10 then we have to find the value of V.
By substituting the given values in the formula,


V = 15 units
Therefore, focal length (V) of the lens = 15 units