Convex Mirror Image Distance and Magnification

[nowtrams]

Homework Statement

At an intersection of hospital hallways, a convex mirror is mounted high on a wall to help people avoid collisions. The mirror has a radius of curvature of 0.510 m.
-What is the image distance for a patient 11.9 m from the mirror? (Use the correct sign conventions.)
-Determine the magnification.

radius of curvature(R)=-0.51m, so the focal length(f)=-0.255m
(negative because the mirror is convex)

object distance(p)=-11.9m

image distance(q)= ?

Homework Equations

1/p + 1/q = 1/f or 1/q = 1/f - 1/p

M = -(q/p)

The Attempt at a Solution

1/q = 1/(-0.255) - 1/(-11.9)
1/q = -1/0.255 + 1/11.9 = -1370/357

q/1 = -357/1370 = -0.2605839 => q = -0.26m


This problem was for a web assignment. I had 5 chances to get the right answer but I used most of them up by using f = 0.225 (+and -) instead of f= -0.255. The feed back for q = -0.26 was that it was close to the correct answer and probably off due to a rounding error. I double checked my calculations but I came up with the same answer. I tried
M = -(q/p) = -(-0.2605839/-11.9) = -0.02198781
which was the incorrect sign. When I submitted M = +0.02198781 I got the same response as I did for the first answer. I'm sure once I get the right answer for q my answer for M will be fixed, but I don't know where to go from here

 

[_coyote_]

. I think my answer for q is correct, but I'm not sure why it's incorrect when the feedback said it was close to the right answer. Any help would be appreciated. Thanks.