Answer:
Estimated cost of building project: $8.5 million 
Actual Cost of building: $8.75 million 
Estimated Duration of project completion: 18 months 
Actual completion of project: 18 months
Particulars Amount 
Estimated Cost (A) $8.50 million 
Actual Cost (B) $8.75 million
Balance (C=B-A) $0.25 million 
Since the period of estimation of project completion and actual completion of project is same i.e. 18 months, the contractor has to pay $0.25 million extra.