PSLE 2009 most difficult question

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PSLE 2009 most difficult question

Jim bought some chocolates and gave half of it to Ken. Ken bought some sweets and gave half of it to Jim. Jim ate 12 sweets and Ken ate 18 chocolates. The ratio of Jim's sweets to chocolates became 1 : 7 and the ratio of Ken's sweets to chocolates became 1:4. How many sweets did Ken buy?. Ans: 68 sweets


Let me show you how to solve the most difficult question using the easiest to understand way.

Jim
Sweets - 12: Chocolate
1 : 7
Ken
Sweets : Chocolate - 18
1 : 4

Jim
Chocolate = 7u x (Sweets -12)

Ken
4u Sweets = Chocolate - 18

combine two, (substitute chocolate into Ken's equation)
4u Sweets = 7u x (Sweets -12) -18
4u Sweets = 7u Sweets - 84-18
3u Sweets = 102
1u Sweets = 34

34 x 2 = 68 sweets bought


This is the simultaneous substitution concept which makes the problem sum so simple and easy to solve, this my easiest way to explain to my current P5 students how to solve this "national level" Maths problem sum that baffled this year's P6 students.