In a HCP unit-cell, if total number of atoms per unit cell gets halved, then what is the percentage void in unit cell ?

Explanation

To solve the problem step by step, we will analyze the information provided and apply the relevant concepts of solid-state chemistry.

### Step 1: Understand the HCP Structure
The hexagonal close-packed (HCP) structure is a type of crystal lattice where atoms are packed closely together. In a typical HCP unit cell, there are 6 atoms per unit cell.

**Hint:** Remember that the HCP structure is characterized by a specific arrangement of atoms that maximizes packing efficiency.

### Step 2: Determine the Initial Packing Efficiency
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