Question posted by Christina from Singapore:

Grade/Level: Pri 4

Question solved by Model Method: A watch costs $25 more than a calculator. 3 such watches cost as much as 8 such calculators. What is the cost of each calculator?

Answer:

Step 1: This question involves the Comparison Concept and the Repeated Variable Concept. Since A watch costs $25 more than a calculator, we draw 1 box to represent the amount of money a calculator cost and draw an identical box plus another box marked with "$25" to show that the watch cost $25 more than a calculator.

Step 2: Next, we draw another 2 sets of unknown part plus $25 to represent the total of 3 such watches.

Step 3: Then we add in another 2 unknown units to represent the cost of the additional 2 calculators (to make the total 3 calculators).

Step 4: If we compare the top and bottom models, we will discover that the difference between the top model (watches) and the bottom model (calculators) is total of the 3 units of $25. This $75 is actually the difference between the top and the bottom model which is the value of 5 calculators (since 3 watches is equal to 8 calculators).

Thus, the difference between the top model and the bottom model must be equal to 5 calculators.

Thus,

5 units ----------> 3 x $25 = $75

1 unit ----------> $75 / 5 = $15

Therefore, 1 calculator costs $15.

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