Let \( d \) represent the number of rental days.
The total cost to rent a mid-sized car from Rental Company A can be expressed as: \[ \text{Cost}_A = 80 + 35d \]
The total cost to rent a mid-sized car from Rental Company B can be expressed as: \[ \text{Cost}_B = 30 + 45d \]
To find the number of rental days where the costs are equal, we set the two equations equal to each other: \[ 80 + 35d = 30 + 45d \]
Now, we can solve for \( d \):
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Subtract \( 30 \) from both sides: \[ 50 + 35d = 45d \]
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Subtract \( 35d \) from both sides: \[ 50 = 10d \]
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Divide both sides by \( 10 \): \[ d = 5 \]
Now that we have found \( d = 5 \), we can calculate the total cost from either company.
Using Rental Company A: \[ \text{Cost}_A = 80 + 35 \cdot 5 = 80 + 175 = 255 \]
Using Rental Company B: \[ \text{Cost}_B = 30 + 45 \cdot 5 = 30 + 225 = 255 \]
Both companies charge \( 255 \) for 5 rental days.
Thus, the total charged by each company for that number of rental days is: \[ \boxed{255} \]