Since we know the actual river length is 247 miles and that it transfers to 4.75 inches on the map we can divide to find our answer. Dividing will show us how many miles is equivalent to 1 inch on the map.
247/4.75 = 52
This means every 52 miles will appear on the map as 1 inch.
You can also check this answer (you do not have to check) by making an equation that shows the two values and finding how much 1 mile is in inches on the map.
247 (miles) = 4.75 (inches)
Now, we can divide 247 by 247 to give us 1 mile, and what you do to one side of the equation you must do to the other.
247/247 = 4.75/247 (I suggest using a calculator for this part)
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1 mile = 0.01923076923 inches on the map
To continue checking the answer multiply both sides by 52. The inches sides will come out as 1 proving that 52 miles is equivalent to 1 inch on the map.
1(mile)*52 = 0.01923076923(inches)*52
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52 miles = 1 inch
Your answer is now proven to be 52 miles is 1 inch on the scale. I suppose you could also write it as 1 mile is equal to 0.01923076923 inches for the scale, but it is unnecessarily complicated and I'm sure your teacher expects the other and much simpler answer.
Answer: The scale converts 52 miles to 1 inch on the map.
