Introduction:
The straight-line method of depreciation is one of the most commonly used techniques to allocate the cost of an asset over its useful life. It assumes that the asset depreciates evenly over time, resulting in a constant annual depreciation expense. In this method, the cost of the asset is divided by its useful life to determine the yearly depreciation amount.

Given Information:
- On 1 April 2010, Sohanlal purchased a plant costing 60000.
- On 1 October 2010, an additional plant was purchased for 40000.
- On 1 July 2011, another plant was purchased for 20000.
- On 1 January, one-third of the plant purchased on 1 April 2010 was sold for 6000.
- The depreciation rate is 10%.

Calculation:

Step 1: Calculate the Total Cost of the Plant:
To calculate the total cost of the plant, we sum up the costs of all the plants purchased.

Total cost of the plant = Cost of plant purchased on 1 April 2010 + Cost of plant purchased on 1 October 2010 + Cost of plant purchased on 1 July 2011
Total cost of the plant = 60000 + 40000 + 20000
Total cost of the plant = 120000

Step 2: Calculate the Total Depreciation:
To calculate the total depreciation, we need to determine the depreciation amount for each plant and sum them up.

Depreciation amount for each plant = (Cost of the plant * Depreciation rate)/100
Depreciation amount for plant purchased on 1 April 2010 = (60000 * 10)/100 = 6000
Depreciation amount for plant purchased on 1 October 2010 = (40000 * 10)/100 = 4000
Depreciation amount for plant purchased on 1 July 2011 = (20000 * 10)/100 = 2000

Total depreciation = Depreciation amount for plant purchased on 1 April 2010 + Depreciation amount for plant purchased on 1 October 2010 + Depreciation amount for plant purchased on 1 July 2011
Total depreciation = 6000 + 4000 + 2000
Total depreciation = 12000

Step 3: Calculate the Net Book Value:
Net book value is the value of the asset after deducting the accumulated depreciation.

Net book value = Total cost of the plant - Total depreciation
Net book value = 120000 - 12000 = 108000

Conclusion:
The straight-line method of depreciation is a simple and widely used method to allocate the cost of an asset over its useful life. In this case, the total cost of the plant is 120000, and the total depreciation is 12000. Therefore, the net book value of the plant is 108000.

8500