This week’s math tips is about difference concept with assumption method – part 2. This question can be spotted in Primary 5-6 papers which carries 5 marks!
Qns: Peter Participated in a Math competition. There was a total of 35 questions. For each correct answer, 3 marks were awarded. For each wrong answer, 2 marks were deducted. For each question left blank, 1 mark was deducted. Peter did not answer 5 questions and scored a total of 70 marks. How many questions did he answer correctly?
Step 1: Remove 1 condition (the questions left blank)
35 – 5 = 30 (Questions that are correct & wrong)
Step 2: Assume all questions are correct
30 x 3 = 90
Step 3: Add back the marks that was deducted due to leaving blank
70 + 5 = 75 (if there are only 30 questions of correct & wrong)
Step 4: Minus the difference
90 − 75 = 15 (Total difference)
Step 5: Difference in 1 group
3 + 2 = 5 (Add due to one deducting and one adding marks)
Step 6: Find the number of groups
15 ÷ 5 = 3
Step 7: Number of correct questions
30 – 3 = 27
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