An examination consists of 160 questions. One mark is given for every correct option. If a 1/4 mark is deducted for each wrong option and a half mark is deducted for each unanswered question, then a student scores 79. If half a mark is deducted for every wrong option and 1/4 mark is deducted for every unanswered question, the person scores 76. Find the number of correct answers he wrote?
Explanation
To solve this problem, let x be the number of correct answers, y be the number of wrong answers, and z be the number of unanswered questions.
Step 1: Set up the equations
The total number of questions is 160:
x+y+z=160
From the first scoring scenario:
x−0.25y−0.5z=79
From the second scoring scenario:
x−0.5y−0.25z=76
Step 2: Simplify the system
From the first equation, z=160−x−y. Substituting this into the scoring equations:
First equation: x−0.25y−0.5(160−x−y)=79
x−0.25y−80+0.5x+0.5y=79
1.5x+0.25y=159 — (Equation A)
Second equation: x−0.5y−0.25(160−x−y)=76
x−0.5y−40+0.25x+0.25y=76
1.25x−0.25y=116 — (Equation B)
Step 3: Solve for x
Adding Equation A and Equation B eliminates y:
(1.5x+0.25y)+(1.25x−0.25y)=159+116
2.75x=275
x=2.75275=100
Conclusion:
The number of correct answers written by the student is 100, which corresponds to option 2.