Given:
- Magnification (m) = -1
- Distance between object and image (d) = 60 cm

To find:
- Focal length of the lens (f)

Formula:
The magnification of a lens is given by the formula:
m = -d'/d
where d' is the distance between the image and the lens, and d is the distance between the object and the lens.

Solution:
Step 1: Finding the focal length of the lens (f)
Since the magnification (m) is given as -1, we can rewrite the formula as:
-1 = -d'/d
1 = d'/d

Since the distance between the object and the image is given as 60 cm, we can substitute the values into the equation:
1 = d'/60

Simplifying the equation, we can find the value of d':
d' = 60 cm

Therefore, the distance between the image and the lens is 60 cm.

Step 2: Finding the focal length of the lens (f)
Using the lens formula, we have:
1/f = 1/d' - 1/d

Substituting the values of d' = 60 cm and d = 60 cm into the equation:
1/f = 1/60 - 1/60

Simplifying the equation, we get:
1/f = 0
f = infinity

Therefore, the focal length of the lens is infinity.

Step 3: Moving the object 20 cm towards the lens
If the object is moved 20 cm towards the lens, the new distance between the object and the lens would be:
d' = d - 20 cm = 60 cm - 20 cm = 40 cm

Using the lens formula, we can find the new position of the image:
1/f = 1/d' - 1/d

Substituting the values of d' = 40 cm and d = 60 cm into the equation:
1/f = 1/40 - 1/60

Simplifying the equation, we get:
1/f = (3/120) - (2/120)
1/f = 1/120
f = 120 cm

Therefore, if the object is moved 20 cm towards the lens, the image would be formed at a distance of 120 cm from the lens.

Conclusion:
- The focal length of the lens is infinity when the magnification is -1 and the distance between the object and its image is 60 cm.
- If the object is moved 20 cm towards the lens, the image would be formed at a distance of 120 cm from the lens.