An object thats is 10 cm tall produces an image that is 5 cm tall is seens in a convex

[Hitsatsu]

Since we have the object height, the image height, and the image distance, we can solve for the object distance:

(hi / ho) = (-di / do)
(5 / 10) = (-(-15) / do)
(1 / 2) = (15 / do)
do = 2(15)
do = 30cm

Now that we have the object distance, we can solve for the focal point:

(1 / f) = (1 / do) + (1 / di)
(1 / f) = (1 / 30) + (1 / -15)
(1 / f) = (1 / 30) + (2 / -30)
(1 / f) = -1 / 30
-f = 30
f = -30cm

The focal point is 30cm behind the convex mirror.

Hope this helped :-]