Question posted by Rosa Brown from Singapore:
Grade/Level: Primary 6
Question solved by Model Method: David and Amrita had an equal number of marbles. After Amrita gave 50 marbles to David, he had 5 times as any marbles as her. find the total number of marbles they had?
Step 1: For this question, it will be easier to start from the "After" model and work backwards to the "Before" model. We first draw 5 boxes to represent the number of marbles David had and 1 box to represent the number of marbles Amrita had after the transfer of 50 marbles.
Step 2: This question is based on the Constant Total Concept . Since the total number of units is 6 units, each of them should have 3 units before the transfer so that they had an equal number of marbles. So we draw 2 boxes in dotted lines to show that they had been transferred from Amrita to David.
Step 3: So the 2 boxes in dotted lines must represent the 50 marbles transferred. We can also draw another vertical bracket to the right of the model to show the total number of marbles both had which is equal to 6 units.
2 units ---------- 50 marbles
1 unit ---------- 50 marbles / 2 = 25 marbles
6 units ---------- 25 marbles x 6 = 150 marbles
Therefore they had 150 marbles in all.