Question posted by Steven from Singapore:

Grade/Level: Primary 5

Question solved by Model Method: Susie bought 5 kg of flour and 4 kg of sugar for $14.80. If 3/4 kg of flour cost as much as 3/5 kg of sugar, find the cost of 1 kg of sugar.

Answer:

Step 1: Draw 4 small boxes to represent the cost of 4 quarters (or 1 whole) of 1 kg of flour. Since 3/4 kg of flour cost as much as 3/5 kg of sugar, mark out 3 boxes (which is 3/4 of 1 kg of flour) to equate to 3 boxes (which is 3/5 of 1 kg of sugar) of sugar. Add 2 small boxes to the right of the "sugar" model to show the cost of the rest (2/5 kg of the sugar) of the 1 kg of sugar.

Step 2: Since 4 small boxes represent the cost of 1 kg of flour, we draw an additional 16 boxes to the right of the "flour" model to represent the rest of the 4 kg of flour she bought arriving at the total of 5 kg of flour bought.

Step 3: Since 5 small boxes represent the cost of 1 kg of sugar, we draw an additional 15 boxes to the right of the "sugar" model to represent the rest of the 3 kg of sugar she bought arriving at the total of 4 kg of sugar bought.

Step 4: Draw a bracket to the extreme right to mark out the total cost of 5 kg of flour and 4 kg of sugar which is $14.80 (given in the question).

From the model,

20 units(flour) + 20 units(sugar) ----------> $14.80

40 units(flour + sugar) ----------> $14.80

1 unit ----------> $14.80 / 40 = $0.37

5 units ----------> $0.37 X 5 = $1.85

Therefore, 1 kg of sugar costs $1.85.