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The first mistake I noticed is in the units.
Then I noticed the error in 640, before the words "check again!" got my attention.
On further reading and thinking...
Physics tutor Phang Yu Hon has nearly 90 students and earns nearly 20,000 Singapore dollars (15,000 US dollars) a month, the report said. The 41-year-old gave up his research engineering job in 1994.Probably without word of mouth, I imagine can get ~20 students and monthly ~S$ ... 4000 is quite nice. All thanks to the high pressure by teachers, who unshamefully asking parents to get tutors for their kids. Anyone interested for a mathematics tutor may check out my answers at Yahoo!Answers.
Sue and Julie were running equally fast around a track. Sue started first. When she had run 9 laps, Julie had run 3 laps. When Julie completed 15 laps, how many laps had Sue run?Link to my experiments: Does your answer make sense? (for students)? and Quick, how many laps Sue ran?