Renting Bike Safety EquipmentDate: 10/09/2001 at 17:34:45 From: Jessica Subject: Solving equations with variables on both sides Can you please help me with problems like: 5r + 6 = 14r - 3? Also, can you help me on this word problem: Susppose you live near a park that has a bike trail you like to ride. The Park Department rents a bike with safety equipment for $5 a day. If you provide your own safety equipment, the bike rental is $3 a day. You could buy equipment at a sports store for $28. How many times must you use the trail to justify buying your own safety equipment? I have no idea how to even start these problems. Jessica Date: 10/10/2001 at 14:09:38 From: Doctor Ian Subject: Re: Solving equations with variables on both sides Hi Jessica, I think this entry from the Dr. Math archives will help you see how to deal with equations that have the same variable appearing on both sides: Basic Tips on Solving for X http://mathforum.org/dr.math/problems/megan.11.16.00.html As for the story problem, it's better to come up with a 'dumb' solution that you understand than try to come up with a tricky one that you have to sort of hope turns out to be right. If you buy equipment, it costs you $28 + $3 = $31 to go riding the first time, and then $3 more for each time after that. Times riding Total cost ------------ ---------- 1 31 2 34 3 37 4 40 5 43 6 46 On the other hand, if you rent equipment, it costs you $5 the first time, and $5 each time after that: Times riding Total cost (buy) Total cost (rent) ------------ ---------------- ----------------- 1 31 5 2 34 10 3 37 15 4 40 20 5 43 25 6 46 30 If you keep adding rows to this table, you'll eventually get to a point where the cost of renting is higher than the cost of buying. That's the number you're looking for. Once you've found an answer that you know must be correct, _then_ you can start thinking about easier ways to find it. For example, each time you rent instead of buying, it costs you an extra $2. So a different way to ask the question is: If you spend an extra $2 each time you rent safety equipment, how many times will you have to rent it before you've spent $28? A third way is to just write down the equations for the two situations, cost of buying = 28 + n*3 cost of renting = 0 + n*5 set the two costs equal to each other, 28 + n*3 = 0 + n*5 and solve this equation to find the value of n that represents the 'break even' point. (So probably this story is supposed to get you to set up the kind of equation that you asked about in the first part of your message.) Note that each of the two original equations, cost of buying = 28 + n*3 cost of renting = 0 + n*5 is the equation of a line. If you plotted the lines (with the number of times on the trail along the x-axis, and the total cost along the y-axis), you'd find that they intersect at a point. The value of n at this point is the number of times that you have to go riding to reach the 'break even' point. All of these approaches should lead you to the same answer - and in fact, finding a single answer in (at least) two completely different ways is one of the best techniques to use for making sure that you've worked a problem correctly. I hope this helps. Write back if you'd like to talk about this some more, or if you have any other questions. - Doctor Ian, The Math Forum http://mathforum.org/dr.math/ |
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