A flight instructor charges $50 per lesson, plus an additional fee for the use of his plane. The charge for the use of the plane varies directly with the square root of the time the plane is used. If a lesson plus 16 minutes of plane usage costs $90, what is the total amount charged for a lesson having 36 minutes of plane usage?

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Well, it's 40/4 for one minute, or 10 for one minute. You want 36 mins, so you multiply by the square root (6), to get $60, to which you also add the $90 for the lesson (result is $150)

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