Kavya is training for a swimming competition. She wrote down the number of laps she swam on Tuesday, Wednesday and Thursday in her logbook. Some water dripped on the page, so only Tuesday's number can be read: she swam 38 laps on Tuesday. The numbers for Wednesday and Thursday are smudged. She remembers that the average number of laps for Wednesday and Thursday was 6 more than the average number of laps for Tuesday and Thursday.
- How many laps did Kavya swim on Wednesday?
- She also remembers that the average number of laps for Tuesday and Wednesday was 5 more than the average number of laps for the 3 days. How many laps did Kavya swim on Thursday?
Need a hint?
In (a), both averages include Thursday, so compare Wednesday with Tuesday. In (b), find the average of Tuesday and Wednesday first, then work back to the 3-day total.
Worked answer
- (a) Both pairs include Thursday, so the extra 6 in the average comes from Wednesday and Tuesday.
- The average of Wed and Thu is 6 more than the average of Tue and Thu, so Wed and Thu together is 6 × 2 = 12 more than Tue and Thu together.
- So Wednesday is 12 more than Tuesday: 38 + 12 = 50 laps.
- (b) Tuesday and Wednesday together: 38 + 50 = 88 laps, so their average is 88 ÷ 2 = 44.
- The average for the 3 days is 5 less than 44, which is 44 − 5 = 39.
- Total for the 3 days: 39 × 3 = 117 laps.
- Thursday: 117 − 88 = 29 laps.
Answer: 29 laps
The mistake this question tests
Habit
Average and total
Many pupils add only 6 to Tuesday's number in (a), forgetting that an average is the total shared by 2, so the totals differ by 12, not 6.
How to fix it at home →More PSLE whole numbers questions
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See the answer →Practise PSLE whole numbers until the mistakes stop.
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