Garden Area Calculation:
To find the cost of covering the remaining portion of the garden with grass, we first need to calculate the area of the garden.

Given:
Length of the garden = 150m
Width of the garden = 120m

Formula:
The area of a rectangle is calculated by multiplying its length and width.

Calculation:
Area of the garden = Length × Width
= 150m × 120m
= 18,000 square meters

Circular Skating Floor Area:
Next, we need to calculate the area of the two circular skating floors.

Given:
Radius of the first skating floor = 35m
Radius of the second skating floor = 42m

Formula:
The area of a circle is calculated by multiplying pi (π) with the square of its radius.

Calculation:
Area of the first circular skating floor = π × (35m)^2
= 3.14 × 1225m^2
= 3848.5 square meters

Area of the second circular skating floor = π × (42m)^2
= 3.14 × 1764m^2
= 5537.6 square meters

Remaining Portion of the Garden:
To find the remaining portion of the garden, we subtract the areas of the two circular skating floors from the total area of the garden.

Calculation:
Remaining portion of the garden = Area of the garden - Area of the first circular skating floor - Area of the second circular skating floor
= 18,000 square meters - 3848.5 square meters - 5537.6 square meters
= 9,613.9 square meters

Cost of Covering with Grass:
Finally, we need to calculate the cost of covering the remaining portion of the garden with grass at a rate of Rs 1.25 per square meter.

Given:
Rate of grass covering = Rs 1.25 per square meter

Formula:
The cost of covering an area with grass is calculated by multiplying the area with the rate.

Calculation:
Cost of covering the remaining portion of the garden with grass = Remaining portion of the garden × Rate of grass covering
= 9,613.9 square meters × Rs 1.25 per square meter
= Rs 12,017.38

Therefore, the cost of covering the remaining portion of the garden with grass at a rate of Rs 1.25 per square meter is Rs 12,017.38.