Why are you taking a map test in january
Answer: r = C/(2pi)
This is the same as writing 
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Explanation:
pi is some number (approximately 3.14) which means 2pi is also some number (roughly 6.28)
Saying c = 2pi*r means we have 2pi times r, and the result is the circumference c. To isolate r, we'll undo the multiplication. We'll undo it by dividing both sides by 2pi like so
C = 2pi*r
C/(2pi) = r
r = C/(2pi)
in which we can write it like 
So whatever C is, we divide it over 2pi (aka roughly 6.28) to get the radius.
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Extra Info (Optional Section):
As an example, let's say the circumference of the circle is 628 feet. This means the distance around the circle is 628 feet. So C = 628 would lead to...
r = C/(2pi)
r = C/(6.28)
r = 628/(6.28)
r = 100
So the radius would be roughly 100 feet.
Graph and equation both shows the proportional comparison between two quantities
for example, equation y = 4x, this means, the value of 'y' will always be 4 times the value of 'x'
More complex equation such as y = 3x + 5, means that the value of 'y' equals to 5 more triples of value of 'x'
Another example is the conversion graph attached below, it shows the relationship between kilometers and miles. For example, we want to find out how many miles are in 10 kilometers, we would draw a line from the point that shows 10 km towards the graph, then across from the graph to miles, and we'd get a reading of 12 miles.