In a test comprising 50 questions, a student attempts all questions. For every correct answer the student is awarded 1 mark. She will get negative marks for incorrect answers as per the following rule.
1. 0.25 negative mark for each of the first 10 incorrect answer.
2. 0.5 negative mark for each incorrect answer, from the 11th to the 20th.
3. 0.75 negative mark for each incorrect answer, from the 21st.
What is the minimum number of questions that the student should get right to get a non-negative score? Choose the correct choice from the following options
A) 17
B) 18
C) 21
D) 22
E) 23
If 21 answers are correct, the student will score = 21*1= 
<gadget id="calculator">21*1</gadget>
<output>21</output>
21   -(10*0.25 + 10*0.5+9*0.75)=-14.25   This huge difference between Right and Wrong shows that C cant be the answer. Based on this we can also eliminate D and E.   Hence its between A and B.   Let plug in A.   17*1= 
<gadget id="calculator">17*1</gadget>
<output>17</output>
17   -(2.5 + 5 + 13*0.75)= -17.25   Result Negative. Hence A can't be the answer and its 18.
<result>B</result>
In a test comprising 50 questions, a student attempts all questions. For every correct answer the student is awarded 1 mark. She will get negative marks for incorrect answers as per the following rule.
1. 0.25 negative mark for each of the first 10 incorrect answer.
2. 0.5 negative mark for each incorrect answer, from the 11th to the 20th.
3. 0.75 negative mark for each incorrect answer, from the 21st.
What is the minimum number of questions that the student should get right to get a non-negative score?