Mike ($3.75): x
Friends ($3.75): 3x
Mike + Friends ≤ 25
3.75(x) + 3.75(3x) ≤ 25
3.75x + 11.25x ≤ 25
15x ≤ 25
≤
x ≤ 1
Answer: they can rent the paddleboards for 1 hour 40 minutes