Answer
(a) virtual
(b) $69cm$
(c) convex
Work Step by Step
(a) We know that the image is virtual because the image is on the same side of the object and can't be captured on the screen.
(b) As $d_i=1.5d_{\circ}=1.5\times 23cm=34.5cm$
We know that $f=\frac{d_{\circ}d_i}{d_{\circ}+d_i}$
We plug in the known values to obtain:
$f=\frac{23\times (-34.5)}{23-34.5}=69cm$
(c) We know that the lens used is convex because the sign of its focal length is positive.
