My answer is 12.57 for the circ.
Since radius is half the diameter and the formula is c/d = pi, the equation should be written as C/4 = pi.
To find the c, I would need to inverse and write the solution as c = 4 x pi.
1 Like
Awesome job for doing the challenge!
@TK_F, great work attempting the challenge.
You’re answer is incorrect but that’s ok, we tend to learn better from our mistakes so let me try and unpack where you went wrong. 
You are correct that the radius is half of the diameter but if C/D = π then this means that C/2r = π, since you need to replace the diameter with 2x the radius so everything stays balanced. You could write this on one line as C/D = C/2r = π.
The challenge set a radius of 8 but I think you may have started with a diameter of 8 instead.
You’ve then halved that to get a radius of 4, which is ok for the problem you’re now working with.
However, by using C/4 = π you’re saying that the the diameter is 4 and the radius is 2 or C/4 = C/(2 x 2) = π.
With that in mind, give the challenge another go and see how you get on.
I’ve included the answer below so you can check your workings.
Start with a radius of 8, so;
r = 8 or d = 16.
From that you can either use;
C / D = π
or
C / 2r = π
These can then be rewritten as;
C = πD
or
C = 2πr
so your answer would be either:
C = π16
or
C = 2π8 = π16
In real-world decimal numbers, this equals about 50.27 (2 d.p.).
I hope this helps clarify things but if you’re still struggling please do reply to this thread and let me know.
2 Likes
Welp, I tried lol and went the wrong direction. Crap, you did say diameter is double the radius (2r). Unfortunately for me it was 4AM in the morning when I was attempting to solve this and wasn’t thinking straight. No excuses though!..better luck next time to me.
Well I’ll let you off it was 4am 
It’s totally fine to make mistakes when you’re first learning - I even still make them now!
The main thing is to not be disheartened and to keep trying. With practice you can only get better
1 Like