To find the missing repair time corresponding to the cost of $630, we first observe the relationship between the repair time and cost in the table.
From the table, we can see:
- For 2 hours, the cost is $180.
- For 5 hours, the cost is $450.
- For 10 hours, the cost is $900.
- For 20 hours, the cost is $1,800.
Now we can look for a consistent pattern. Let's calculate the cost per hour for the known values:
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For 2 hours: \[ \frac{180}{2} = 90 \text{ dollars per hour} \]
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For 5 hours: \[ \frac{450}{5} = 90 \text{ dollars per hour} \]
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For 10 hours: \[ \frac{900}{10} = 90 \text{ dollars per hour} \]
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For 20 hours: \[ \frac{1800}{20} = 90 \text{ dollars per hour} \]
The cost appears to increase linearly at a rate of $90 per hour.
Since we are looking for the repair time corresponding to a cost of $630, we can set up the following equation based on the cost per hour:
\[ \text{Cost} = \text{Repair Time (hours)} \times 90 \]
Substituting the cost of $630 into the equation:
\[ 630 = \text{Repair Time} \times 90 \]
Now we can solve for the repair time:
\[ \text{Repair Time} = \frac{630}{90} \]
Calculating that gives us:
\[ \text{Repair Time} = 7 \]
Thus, the missing number in the third row of the table is 7 hours.
Therefore, the expression that can be used to find the missing number is:
\[ \text{Repair Time} = \frac{630}{90} \]
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