To find the cost of the border for Gabrielle's garden, we can use the formula provided:
\[ \text{Cost} = 1.50(2l + 2w) \]
Given that the length \( l \) is 4 yards and the width \( w \) is 3 yards, we can substitute these values into the formula:
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Calculate \( 2l + 2w \): \[ 2l + 2w = 2(4) + 2(3) = 8 + 6 = 14 \]
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Now multiply by the cost per yard: \[ \text{Cost} = 1.50 \times 14 = 21.00 \]
Therefore, the cost for the border around Gabrielle's garden is $21.00.