To find the length of the rectangular field, you can use the formula for the perimeter of a rectangle:
\[ P = 2(\text{length} + \text{width}) \]
Given that the perimeter \( P \) is 80 m and the width is 15 m, we can set up the equation:
\[ 80 = 2(\text{length} + 15) \]
Now, divide both sides by 2:
\[ 40 = \text{length} + 15 \]
Next, subtract 15 from both sides to solve for the length:
\[ \text{length} = 40 - 15 \] \[ \text{length} = 25 \text{ m} \]
So, the length of the rectangular field is \( 25 \) m.