Let's denote the cost of one guitar pick as \( c \). According to the problem, the cost of a guitar string is \( c + 1 \).
Patrick purchased 25 guitar picks and 25 guitar strings. Therefore, the total cost can be expressed as follows:
\[ \text{Total cost} = (25 \text{ guitar picks}) \times c + (25 \text{ guitar strings}) \times (c + 1) \]
This can be simplified to:
\[ \text{Total cost} = 25c + 25(c + 1) \]
Let's further simplify:
\[ \text{Total cost} = 25c + 25c + 25 = 50c + 25 \]
According to the problem, he spent a total of $40, hence we set up the equation:
\[ 50c + 25 = 40 \]
Now, let's solve for \( c \):
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Subtract 25 from both sides: \[ 50c = 40 - 25 \] \[ 50c = 15 \]
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Divide both sides by 50: \[ c = \frac{15}{50} = \frac{3}{10} = 0.30 \]
Thus, the cost of one guitar pick is \( $0.30 \).
Now, to find the cost of one guitar string:
\[ \text{Cost of one guitar string} = c + 1 = 0.30 + 1 = 1.30 \]
Therefore, the cost of one guitar string is
\[ \boxed{1.30} \]