Final answer:
The object must be placed 30.0 cm from the concave mirror.
Step-by-step explanation:
To find the object distance, we can use the formula for the mirror's focal length: f = R/2, where R is the radius of curvature of the mirror.
In this case, the mirror's radius of curvature is given as 30.0 cm, so the focal length is f = 30.0/2 = 15.0 cm. When the image is at infinity, the object distance is equal to twice the focal length: do = 2f = 2 × 15.0 cm = 30.0 cm.