Finding distance of image from mirror given focal length

[RabbitWho]

Homework Statement

An object is at a very large distance from a concave mirror of focal length 40cm. Find the distance from the image to the mirror. Is the image real or virtual?

Homework Equations

1/u + 1/v = 1/f

The Attempt at a Solution

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1/u + 1/v = 1/40

The image has to be real because it must be outside the focus if it is very far from the mirror, right?
It seems to me like I haven't got enough information to solve the problem. How would I go about doing it? The answer from the key is 40cm.

 

[RUber]

What do you get if you put in a very large number (say, 10000cm) for the object distance? What happens as that distance gets even farther?