Read the sign of a virtual image

An object is 10 cm in front of a concave spherical mirror of focal length 15 cm. Use the real-positive convention: real object and real image distances are positive.

Find the signed image distance.

Organize the givens & assumptions

Real object distance: +10 cm
Concave mirror focal length: +15 cm
Real-positive distance convention

Paraxial rays and ideal spherical mirror. Negative image distance means a virtual image behind the mirror in this convention.

A $+30\ \mathrm{cm}$
B $+6\ \mathrm{cm}$
C $-6\ \mathrm{cm}$
D $-30\ \mathrm{cm}$

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