Do you know WHEN and HOW to use the Row-Minus-Row Method? This method is used when the question involves several relationships between multiple items and for each relationship a Total is given.

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Question:

Kyle bought 6 similar pencils, 3 similar books and a file for $78.

1 pencil and 1 book cost $20.

1 pencil and 1 file cost $12.

What was the cost of the file?

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Solution using the Row-Minus-Row Method:

Given 6p + 3b + 1f = 78  ———–Row 1

Given 1p + 1b = 20

So 3p + 3b = 60  ———– Row 2

Row 1 – Row 2:  3p + 1f  = 18  ———— Row 3

Given 1p + 1f = 12

So 3p + 3f = 36  ————- Row 4

Row 4 – Row 3:   2f = 18, so f = 9.

Ans:  $9