Raj had 30 more marbles than Ken at first. After Raj gave some marbles to Ken, he had 18 fewer marbles than Ken. How many marbles did Raj give to Ken?
Need a hint?
Draw two bars. Raj starts 30 ahead and ends 18 behind. How big is the total change in the gap? Remember each marble given away changes the gap by 2.
Worked answer
- At first, Raj was 30 marbles ahead of Ken.
- For Raj to end up 18 marbles behind Ken, he must close the 30-marble gap and then fall behind by 18.
- Total swing = 30 + 18 = 48 marbles.
- Each marble Raj gives away changes the gap by 2 (Raj loses 1, Ken gains 1).
- Number given = 48 ÷ 2 = 24.
The mistake this question tests
Calculation slip
Pupils often just do 30 + 18 = 48 and stop, forgetting that giving 1 marble changes the difference by 2, so they must divide by 2.
How to fix it at home →Wrong answers children often give:
- 6 Calculation slip
- 12 Calculation slip
- 48 Calculation slip
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See the answer →Practise PSLE whole numbers until the mistakes stop.
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