Mark appropriate one - type questions

One of the distinct features of Very Short Answer type questions is

One Mark Question

Select and write the most appropriate answer from the given alternatives for each sub-question : Number of types of orthorhombic unit cell is ...........

There are 180 multiple choice questions in a test. If a candidate gets 4 marks for every correct answer and for every unattempted or wrongly answered question one mark is deducted from the total score of correct answers. If a candidate scored 450 marks in the test, how many questions did he answer correctly?