A work-force of 420 men with a contractor can finish a certain piece of work in 9 months. How many extra men must he employ to complete the job in 7 months?

Explanation

To solve the problem step by step, we will use the concept of direct variation.

### Step 1: Understand the given information
We know that 420 men can complete the work in 9 months. We need to find out how many men are required to complete the same work in 7 months.

### Step 2: Set up the equation
Let \( x \) be the number of men required to complete the work in 7 months. According to the work formula:
\[
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