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Leon and his friends have a total of $25.50 and the cost of renting a paddleboat for 1/2 hour is $8.50. Therefore, they can ride in a paddleboat for 25.50/8.50 = <<25.50/8.50=3>>3 sets of 1/2 hour rides.
Hence, they can ride in a paddleboat for 3 x 1/2 = <<3*1/2=1.5>>1.5 hours. Answer: \boxed{1.5}.