To find the number of movies streamed that would result in both streaming plans costing the same, we can set up an equation based on the cost of each plan.
Let \( x \) be the number of movies streamed in a month.
The total cost for Plan A can be expressed as: \[ \text{Cost of Plan A} = 15 + 4x \]
The total cost for Plan B can be expressed as: \[ \text{Cost of Plan B} = 9 + 6x \]
We want to find the value of \( x \) for which the costs of the two plans are equal: \[ 15 + 4x = 9 + 6x \]
Now, we will solve this equation step by step:
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Subtract \( 4x \) from both sides: \[ 15 = 9 + 6x - 4x \] \[ 15 = 9 + 2x \]
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Subtract 9 from both sides: \[ 15 - 9 = 2x \] \[ 6 = 2x \]
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Divide both sides by 2: \[ x = 3 \]
Therefore, the number of movies streamed that would result in both plans costing the same amount is \( \boxed{3} \).