Final answer:
To determine the unknown digit in the total cost of pizzas ordered, one must realize that since the total cost ends in '25' and is a multiple of 9, it implies that each pizza costs an amount ending in '5.' By using this and the divisibility rule for 9, we discover that the unknown digit is '3' and each pizza costs $15.
Step-by-step explanation:
The key to solving this problem is to find the unknown digit represented by '25,' where '5' is the unknown digit, in the total cost before taxes for the pizzas. Since 9 pizzas are being ordered and the total cost is a number that ends in '25,' we can deduce that the cost of each pizza must also end in a '5' or a '0' because when you multiply a number by 9, the resulting unit digit must be the same as the original unit digit or zero. However, since we know the total ends in '25,' it cannot end in '0,' therefore the cost per pizza ends in '5.'
We then use the fact that the total cost is a multiple of 9 to find the unknown digit. If we assume that the unknown digit is 'x,' then the equation would be 9 * (pizza cost) = (2x + 25). To find the value of 'x,' we must find a number that when added to 2, the last digit of the result is a '2' or '7' (because the total should be divisible by 9), which is '3' (since 2 + 3 = 5 and 5 or 50 is divisible by 9). Therefore, the unknown digit 'x' is 3, making the total cost before taxes $135, and each pizza costs $15.