What are dimensionless variables? Why are they called so?

Explanation

### Step-by-Step Solution:

1. **Definition of Dimensionless Variables**:
- Dimensionless variables are quantities that do not have any physical dimensions associated with them. This means they are expressed as pure numbers without any units.

2. **Explanation of Why They Are Called Dimensionless**:
- They are called dimensionless because they are derived from the ratio of two quantities that have the same dimensions, resulting in the cancellation of those dimensions. For instance, when you take the ratio of length to length, the dimensions cancel out, leaving a pure number.

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