Answer:
Focal length is 6.67 cm.
Step-by-step explanation:
Object distance, u, from the lens = 10 cm
Image distance, v, to the lens = 10 cm + 10 cm
= 20 cm
Apply the lens formula to determine the focal length, f, of the lens.
=
+

=
+

=

=

Find the inverse of both sides, so that;
f =

= 6.67
f = 6.67 cm
The focal length of the lens is 6.67 cm.