Answer
The focal length(f) = +8.6 cm.
Work Step by Step
The image distance is given by
$i = -mp$
By substituting m = -0.40 and p = +30 cm, we find:
$i=-(-.4)(30)= +12 cm$
By substituting i = +12 cm and p = +30 cm into the equation, we obtain:
$1/f = 1/p + 1/i\\\\
1/f = 1/30 + 1/12\\\\
f = +8.6 cm$
