To solve for the length of \( DB \), we start with the information given in the problem:
- The triangles \( AABC \) and \( AEDC \) are equal, indicating they have the same dimensions or area.
- It is stated that point \( C' \) is the midpoint of segments \( AE \) and \( DB \).
- Boat A is \( 5 \) miles from Boat B and \( 30 \) miles from point E.
Given that \( C' \) is the midpoint of both segments, we can express lengths as follows:
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Let \( AE = x \) and \( DB = y \).
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Since \( C' \) is the midpoint, we know:
\[ C'A = C'E = \frac{x}{2} \] \[ C'D = C'B = \frac{y}{2} \]
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Knowing Boat A is \( 30 \) miles from \( E \), it can be represented as the distance of \( AE \).
Additionally, we know Boat A is \( 5 \) miles from Boat B, which can be geometrically positioned in relation to \( DB \).
Now, using the information about the distances:
- Boat A is \( 30 \) miles away from E — implying \( x = 30 \).
- Boat B is \( 5 \) miles away from Boat A, which would suggest that if we place them in a line (keeping geometry in mind), \( DB \) will need to account for that distance.
According to the formulas, if \( AE = 30 \):
- For \( C' \): \[ C'A + C'D = 30 + y/2 \]
Since \( C' \) splits both segments \( AE \) and \( DB \), we can set a relationship:
Using the midpoint and equal triangles (implying equal segments) we would assume a result that compares \( 30 - 5 = 25 \). But for \( DB\), we also take half positions into account, which were not directly involved but can round out our estimations.
Using practical deduction: Given the problem suggests multiple-choice hints with approximate clues, by understanding that the distance relationships likely indicate what's nearest to complete \( y \).
Since:
- \( DE \) can be directly attributed to \( B \) in spatial geometry positions derived from the relationships, we can estimate proportions that lead to derive, thus breaking it down within \( 30y\).
Checking available answers:
- Best approaching \( DB = 25\) would satisfy our configurations based on both midpoint relations and equidistances previously described.
Thus confirming: The length of \( DB \) is 25 miles.