Eric and Deborah are partners at a law firm. They are trying to determine which of them has a comparative advantage in typing the 25 pages required for a sales pitch to a prospective client. Eric can type 20 pages per hour. For other activities, he can bill clients $500 per hour. Eric's opportunity cost of typing pages isper page.
Deborah's opportunity cost of typing pages is 25% lower than Eric's. However, as the senior partner, her billing rate is 20% higher. Based on all of these facts,has a comparative advantage in typing pages.

Eric's opportunity cost of typing pages is $25 per page.

Based on all of these facts, Deborah has a comparative advantage in typing pages.

Explanation:

Eric's opportunity cost of typing is $500 / 20 pages = $25 per page.

Since's Deborah's opportunity cost of typing pages is 20% less than Eric's, then she has a comparative advantage in typing pages.

The person, business or country with the lowest opportunity cost has the comparative advantage in producing that good.

r = (C / π) / 2

Actually the formula of the circumference is Pi multiplied to diameter; where the diameter is equal to twice the radius.

C = π d
d = 2 r
C = π * 2r

If we need to find the radius from the Circumference then

r =  (C / π) / 2

You must differentiate Pi from the number 2 because 2 is part of the radius. This is also to avoid confusion.

For example:  r = 5 ; π - 3.14; find the circumference

C = π * 2r
C = 3.14 * (2*5)
C = 3.14 * 10
C = 31.40

If the circumference is given and the radius is missing, then,

r = (C/π) / 2
r = (31.4 / 3.14) / 2
r = 10 / 2
r = 5