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To solve this problem, let's break down the information given and identify what is necessary to determine the total bill amount.

1. **Information Provided:**
   - The customer left $0.90 as a tip.
   - The tax was 4%.
   - The tip was 10% of the after-tax cost.

2. **Key Question:**
   - Which information is not needed to compute the bill after tax and tip?

3. **Analysis:**
   - Since the tip is given as $0.90, and this amount represents 10% of the after-tax cost, we can use this information directly to find the after-tax cost.
   - Let the after-tax amount be $ x $. Then, 10% of $ x $ (which is $ 0.10x $) should equal $0.90.
   - Solving for $ x $, we get:
     \[
     0.10x = 0.90
     \]
     \[
     x = \frac{0.90}{0.10} = 9.00
     \]
   - So, the after-tax cost is $9.00.
   - To find the original bill before tax, we know that the tax is 4%, so the original amount (before tax) is calculated as follows:
     \[
     \text{Original amount} = \frac{9.00}{1.04} = 8.65 \text{ (approximately)}
     \]

4. **Determining Which Information is Unnecessary:**
   - Since we do not need the specific tax rate of 4% to determine the tip (only the amount of $0.90 was needed), option **A** ("The tax was 4%.") is the correct answer for the information that is not needed.

5. **Total Bill Calculation:**
   - The total bill after tax and tip is:
     \[
     9.00 + 0.90 = 9.90
     \]
   - So, the total bill is **$9.90**.

Would you like further details or explanations on any part of this solution?

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Here are some related questions to deepen your understanding:

1. How would the total change if the tax was 5% instead of 4%?
2. If the customer had left a different tip amount, say $1.00, how would the calculations change?
3. Why is the tax percentage irrelevant for determining the total bill?
4. Can we calculate the before-tax amount using only the tip percentage and tip amount?
5. How would the solution differ if the question asked for the original bill amount without any tax?

**Tip:** When given percentages in problems, identify which values directly affect the calculations and which ones might be extraneous for the solution.

A restaurant customer left $0.90 as a tip. The tax was 4% and the tip was 10% of the after-tax cost. Which information is not needed to compute the bill after tax and tip? What was the total bill?

Learn how to calculate a restaurant bill after tax and tip, given a specific tip amount and percentage. This problem demonstrates percentage calculation and is suitable for students in Grades 6-8.

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