Explanation

Find the area of the rectangular lawn:

Area = Length x Breadth = 80m x 60m = 4800 square meters

Since there are two roads, one parallel to the length and one parallel to the breadth, we need to find the area of each road:

Road parallel to length:

Area = Length x Width = 80m x 10m = 800 square meters

Road parallel to breadth:

Area = Breadth x Width = 60m x 10m = 600 square meters

However, the problem states that the roads are running in the middle of the lawn, which means they intersect. To avoid double-counting the intersection area, we need to subtract it:

Intersection Area = Width x Width = 10m x 10m = 100 square meters

Now, add the areas of both roads and subtract the intersection area:

Total Road Area = (800 + 600) - 100 = 1300 square meters

Finally, calculate the cost of gravelling the roads at Rs. 30 per square meter:

Cost = Total Road Area x Rate = 1300 x 30 = Rs. 39000