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 71%?
M13-03.
Choose the correct choice from the following options:
A) N−35  B) N+72  C) 2N+70  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  0.71*200= 
<gadget id="calculator"> 0.71*200</gadget>
<output>142</output>
142 correct answers, so in the second test he must get   142-70= 
<gadget id="calculator">  142-70</gadget>
<output>72</output>
72 questions correctly.   Now, plug N=1 into the answer choices to see which one yields 72. Only option D fits.   Answer: 2N+70.
<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 71%?
M13-03.