Focal Length of the Concave Lens:
Given that the focal length of the concave lens is 20 cm.

Object Distance:
We need to find the distance from the lens at which a 5 cm tall object should be placed to form an image at 15 cm from the lens.

Using the Lens Formula:
The lens formula is given by:

1/f = 1/v - 1/u

Where,
f = focal length of the lens
v = image distance
u = object distance

Finding the Object Distance:
Given that the focal length (f) is -20 cm (since it is a concave lens) and the image distance (v) is 15 cm, we can rearrange the lens formula to find the object distance (u):

1/(-20) = 1/15 - 1/u

Simplifying the equation:

-1/20 = 1/15 - 1/u

Combining the fractions:

-3/60 = 4/60 - 1/u

Simplifying further:

-3/60 = (4-1)/60

-3/60 = 3/60

Therefore, the object distance (u) is -60 cm.

Calculating the Size of the Image:
To calculate the size of the image, we can use the magnification formula:

magnification (m) = height of the image (h') / height of the object (h)

Given that the height of the object (h) is 5 cm, we can substitute the values into the formula:

m = h' / 5

We know that the magnification (m) is also equal to the ratio of the image distance (v) to the object distance (u):

m = v / u

Substituting the values:

m = 15 / -60

Simplifying:

m = -1/4

Therefore, the magnification is -1/4.

Now, we can use the magnification formula to find the height of the image (h'):

-1/4 = h' / 5

Simplifying:

h' = -1.25 cm

Therefore, the height of the image is -1.25 cm.

Summary:
To summarize, when a 5 cm tall object is placed at a distance of -60 cm from a concave lens with a focal length of 20 cm, an inverted image is formed at a distance of 15 cm from the lens. The size of the image is -1.25 cm.