After taking N tests, each containing 100 questions, John had an average of 70% of correct answers. How much does John need to score on the next test to make his average equal 75%?
M13-03. Choose the correct choice from the following answers.
A) N−35
B) N+72
C) 2N+78
D) 2N+72
E) 2N−35
Say N=1.   So, after 1 test John has 70 correct answers.   In 2 tests, so in 200 questions he needs to have  0.75*200= 
<gadget id="calculator"> 0.75*200</gadget>
<output>150</output>
150 correct answers, so in the second test he must get   150-70= 
<gadget id="calculator">  150-70</gadget>
<output>80</output>
80 questions correctly.   Now, plug N=1 into the answer choices to see which one yields 80. Only option D fits.   Answer: 2N+78.
<result>C</result>
After taking N tests, each containing 100 questions, John had an average of 70% of correct answers. How much does John need to score on the next test to make his average equal 75%?
M13-03.