A narrow uniform glass tube contains air enclosed by 15 cm long thread of mercury. When the tube is vertical with open end uppermost, the air column is 30 cm long. When the tube is inverted, the length of air column becomes 45 cm. Calculate the atmospheric pressure.

Explanation

To solve the problem, we need to analyze the situation step by step.

### Step 1: Understand the setup
- We have a glass tube with a mercury thread that is 15 cm long.
- When the tube is vertical, the air column is 30 cm long.
- When the tube is inverted, the air column becomes 45 cm long.

### Step 2: Analyze the vertical position
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