An object is placed in front of a concave mirror and it produces a real image of magnification 1.5. If the object distance is decreased by 5 cm, the magnification of the real image is increased to 4. Find the focal length of the concave mirror.

Explanation

To solve the problem step by step, we will use the mirror formula and the magnification formula for concave mirrors.

### Step 1: Understand the given information
- The magnification (m) for the first case is -1.5 (real image).
- The object distance (u) is decreased by 5 cm in the second case, and the new magnification (m') is -4.

### Step 2: Write the magnification formulas
For a concave mirror, the magnification is given by:
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