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The correct answer is $5$. It's given that the equation $10x+15y=85$ represents the situation, where $x$ is the number of on-site training courses, $y$ is the number of online training courses, and $85$ is the total number of hours of training courses the apprentice has enrolled in. Therefore, $10x$ represents the number of hours the apprentice has enrolled in on-site training courses, and $15y$ represents the number of hours the apprentice has enrolled in online training courses. Since $x$ is the number of on-site training courses and $y$ is the number of online training courses the apprentice has enrolled in, $10$ is the number of hours each on-site course takes and $15$ is the number of hours each online course takes. Subtracting these numbers gives $15-10$, or $5$ more hours each online training course takes than each on-site training course.