To find the missing factor \( A \) in the equation \( 36,000 ÷ A = 90 \), we can rearrange the equation to solve for \( A \):
\[ A = \frac{36,000}{90} \]
Now, we can simplify \( \frac{36,000}{90} \) using mental math:
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First, simplify \( \frac{36,000}{90} \) by dividing both the numerator and denominator by 10: \[ \frac{36,000 \div 10}{90 \div 10} = \frac{3,600}{9} \]
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Next, divide \( 3,600 \) by \( 9 \).
- \( 3,600 \div 9 \) can be simplified; first, we can calculate: \[ 3,600 = 3,590 + 10 = 3,600 - 9 = 3,590 - 2 = 599 - 1 \]
- However, we know \( 9 \) goes into \( 36 \) exactly \( 4 \) times with a remainder, so:
Therefore: \[ 600 ÷ 9 = 400 \]
Putting this together:
- \( 3,600 ÷ 9 = 400 \)
Thus, we find that \( A = 400 \).
The correct answer is \( C \) \( 400 \).