**Plant and Machinery Account**

Plant and Machinery is a fixed asset that is used in the production process and has a long life span. It is subject to wear and tear and therefore needs to be depreciated over time. In this case, we are required to prepare the Plant and Machinery Account from 1991 to 1993, taking into consideration the purchases and sale of machinery, and providing depreciation at 10% per annum on the straight-line method.

**Straight Line Method of Depreciation:**

The straight-line method of depreciation is the simplest and most commonly used method for allocating the cost of an asset over its useful life. Under this method, the depreciation expense is the same every year, and it is calculated by dividing the cost of the asset by its useful life.

The formula for calculating depreciation using the straight-line method is:

Depreciation Expense = (Cost of Asset - Residual Value) / Useful Life

Where:
- Cost of Asset: The original cost of the asset
- Residual Value: The estimated value of the asset at the end of its useful life
- Useful Life: The estimated number of years the asset will be used

Now let's prepare the Plant and Machinery Account from 1991 to 1993, considering the given information and applying the straight-line method of depreciation.

**Plant and Machinery Account from 1991 to 1993**

| Date | Particulars | Amount (Rs.) | Depreciation (Rs.) | Cost (Rs.) |
|------------|----------------------------|--------------|--------------------|------------|
| 01/01/1991 | To Purchases | 120,000 | - | 120,000 |
| 01/07/1991 | To Purchases | 50,000 | - | 50,000 |
| 01/04/1992 | To Purchases | 25,000 | - | 25,000 |
| 01/10/1993 | By Sale | - | -2,000 | -18,000 |
|------------|----------------------------|--------------|--------------------|------------|
| | Total | 195,000 | -2,000 | 177,000 |
|------------|----------------------------|--------------|--------------------|------------|

**Depreciation Calculation:**

Depreciation is calculated as follows:
- Depreciation Expense = (Cost of Asset - Residual Value) / Useful Life
- Depreciation Expense = (120,000 - 0) / 10 = 12,000 per year

**Depreciation Expense for each year:**

| Year | Opening Balance (Rs.) | Depreciation Expense (Rs.) | Closing Balance (Rs.) |
|------------|----------------------|----------------------------|-----------------------|
| 1991 | 120,000 | 12,000 | 108,000 |
| 1992 | 108,000 | 12,000 | 96,000 |
| 1993 | 96,000 | 12,000 | 84,000 |

**Plant and Machinery Account (after providing depreciation):**

| Year | Particulars | Amount (Rs.) |
|------------|--------------------------|--------------|
| 1991 | To Depreciation |