That is not difficult.
The area of the garden is S=A-B
A is the area of the square: A=25 x 25=625
B is the area of the house: B= 20x15= 300
So we have S=625-300=325
With the known ratio, we can calculate the cost: 55*S= 55*325= 17,875
We have to pay Rs 17,875
Have fun