Let \( x \) represent the number of toppings added to the pizza.
The cost of a large cheese pizza at Peter's Pizzeria can be expressed as: \[ C_P = 6.80 + 0.90x \]
The cost of a large cheese pizza at Gavin's Pizzeria can be expressed as: \[ C_G = 7.30 + 0.65x \]
To find the number of toppings needed for the pizzas to cost the same, we set the two cost equations equal to each other: \[ 6.80 + 0.90x = 7.30 + 0.65x \]
Now, we will isolate \( x \).
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Subtract \( 0.65x \) from both sides: \[ 6.80 + 0.90x - 0.65x = 7.30 \] \[ 6.80 + 0.25x = 7.30 \]
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Subtract \( 6.80 \) from both sides: \[ 0.25x = 7.30 - 6.80 \] \[ 0.25x = 0.50 \]
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Divide both sides by \( 0.25 \): \[ x = \frac{0.50}{0.25} = 2 \]
Thus, the number of toppings needed for the pizzas to cost the same is \( \boxed{2} \).