direct and inverse proportion help needed!

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lcrabt21
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Ok so I suck at proportion so I need help with the following questions (if you have a method that works for them it's be greatly appreciated because everything I have tried has failed

If a 5500N force can stop the car in 8.0s, how much force would be needed to stop it in 3.5s? (I managed to get 9400 but the online quiz thing i'm using said it was wrong) (NOW SOLVED)

The current through a resistor is inversely proportional to its resistance. With a 330Ω resistor, the current is 25mA. What value of resistance is needed if you wish a 55mA current to pass? (I have attempted this but the method I am using came back with the incorrect answer)

An interest free loan for a luxury sofa takes 15months to pay back at £80/month.
Monthly charge ∝ number of sofas/duration of loan.
What would the monthly charge be in pounds if I bought 3 sofas and paid for them over one year? Give your answer to the nearest pound. (again I have attempted this a couple of times but the method I use is gave me the incorrect answer)

If anyone has a method and can help me it would be greatly appreciated! I was attempting to use the A=k/B rule but it didn't work out too well for these questions
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the bear
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acceleration = V/t

in both cases V is the same so A1/A2 = {V/8)/{V/3.5}

the force required is directly proportional to the acceleration, so the ratio of the forces is the same as the ratio of the accelerations ie 3.5: 8
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absolutelysprout
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moving to the maths forum
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lcrabt21
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(Original post by entertainmyfaith)
moving to the maths forum
Thank you!
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lcrabt21
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(Original post by the bear)
acceleration = V/t

in both cases V is the same so A1/A2 = {V/8)/{V/3.5}

the force required is directly proportional to the acceleration, so the ratio of the forces is the same as the ratio of the accelerations ie 3.5: 8
I get where you are coming from and this explanation really helped me! but i still seem to be getting it wrong :/ could you possibly explain a little further on how this works??? thank you!!!
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the bear
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(Original post by lcrabt21)
I get where you are coming from and this explanation really helped me! but i still seem to be getting it wrong :/ could you possibly explain a little further on how this works??? thank you!!!
so the ratio of the accelerations is 3.5:8

the ratio of the forces is also 3.5:8

so 5500: ??? is 3.5: 8
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lcrabt21
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Ahh thank you so much!!! I had the correct answer once before but I didn't realise it had to be to a certain significant figure! Thank you so much for your help on that question!!!
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the bear
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(Original post by lcrabt21)
Ahh thank you so much!!! I had the correct answer once before but I didn't realise it had to be to a certain significant figure! Thank you so much for your help on that question!!!
your welcome

:hat2:
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MysticImogen
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(Original post by lcrabt21)
Ok so I suck at proportion so I need help with the following questions (if you have a method that works for them it's be greatly appreciated because everything I have tried has failed

If a 5500N force can stop the car in 8.0s, how much force would be needed to stop it in 3.5s? (I managed to get 9400 but the online quiz thing i'm using said it was wrong)

The current through a resistor is inversely proportional to its resistance. With a 330Ω resistor, the current is 25mA. What value of resistance is needed if you wish a 55mA current to pass? (I have attempted this but the method I am using came back with the incorrect answer)

An interest free loan for a luxury sofa takes 15months to pay back at £80/month.
Monthly charge ∝ number of sofas/duration of loan.
What would the monthly charge be in pounds if I bought 3 sofas and paid for them over one year? Give your answer to the nearest pound. (again I have attempted this a couple of times but the method I use is gave me the incorrect answer)

If anyone has a method and can help me it would be greatly appreciated! I was attempting to use the A=k/B rule but it didn't work out too well for these questions
Thought you asked direct and inverse population control at first!
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Muttley79
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(Original post by lcrabt21)
Ok so I suck at proportion so I need help with the following questions (if you have a method that works for them it's be greatly appreciated because everything I have tried has failed
Could you post your working so we can see where you are going wrong?

One way that something helps is the unitary approach.

A ship has supplies to last 10 men thirty days. How many days will the supplies last a crew of 6?

10 men thirty days
I man = 10 x 30 days
6 men = 10 x 30/6 = 50 days.
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