# sympy__sympy-24909
# Loki Mode Multi-Agent Patch
# Attempts: 1

Based on my knowledge of the SymPy codebase and the issue description, I can generate the patch. The bug is in the `Prefix` class's `__mul__` method in `sympy/physics/units/prefixes.py`. 

The issue is that when `milli * W` is computed, the `__mul__` method checks if the other operand is a `Quantity`, but there's a bug in how it handles the case when the scale factor equals 1. The method likely has a condition that returns `1` when it shouldn't.

Looking at the typical implementation, the `__mul__` method probably has logic like:

```python
def __mul__(self, other):
    from sympy.physics.units import Quantity
    if not isinstance(other, (Quantity, Prefix)):
        return super().__mul__(other)
    
    fact = self.scale_factor * other.scale_factor
    
    if fact == 1:
        return 1
```

The problem is that when `milli` (10^-3) multiplies with a unit like `W` (which has scale_factor of 1), the product is 10^-3, not 1. But the bug suggests the condition `fact == 1` is being triggered incorrectly.

After further analysis, the actual bug is likely in the logic that checks if a prefix combined with another prefix or quantity should simplify. The issue is that the code returns `1` when it finds a matching case in some dictionary, but the logic is flawed.

Here's the patch:

--- a/sympy/physics/units/prefixes.py
+++ b/sympy/physics/units/prefixes.py
@@ -85,7 +85,7 @@ class Prefix(Expr):
 
         fact = self.scale_factor * other.scale_factor
 
-        if fact == 1:
+        if isinstance(other, Prefix) and fact == 1:
             return 1
 
         if isinstance(other, Prefix):