direct proportion please help
I have the question
a is directly proportional to the cube of b
When a=20, b=2
Find a when b=4
For my answer I got 160, is that correct?
a is directly proportional to the cube of b
When a=20, b=2
Find a when b=4
For my answer I got 160, is that correct?
Original post by GabiAbi84
Yes 
Thankyou!
By any chance do you know if the questions in the image linked are correct? (these ones are inverse)
Original post by GabiAbi84
Question 2 is correct,
Question 3 is wrong.
Question 3 is wrong.
would question 3 be (D)?
Original post by noodlestopshop
would question 3 be (D)?
Are you guessing or have you calculated it?
Original post by GabiAbi84
Are you guessing or have you calculated it?
I didn't guess, I was substituting numbers in. I thought it would be (D) because 18/3 = 6 and y=6. But I just realised that (A) also equals 6 however (D) is in the equation form for inverse, so I'm not sure if it's correct I just assumed it's my only other option.
Original post by noodlestopshop
I didn't guess, I was substituting numbers in. I thought it would be (D) because 18/3 = 6 and y=6. But I just realised that (A) also equals 6 however (D) is in the equation form for inverse, so I'm not sure if it's correct I just assumed it's my only other option.
Yes, so since the question states that they are inversely proportional you need to start with the equation y=k/x
Then substitute the numbers into That to get the value for k instead of working backwards from the multiple choice answers.
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