Answer:

Step-by-step explanation:
Given that radius of the circle = 10
Using π, we need to find about which equation expresses the ratio of the circumference of the circle to the circle's diameter.
diameter of the circle = 2(radius) = 2(10)=20
circumference of the circle = 2 π r = 2 π (10) = 20 π
then ratio of the circumference of the circle to the circle's diameter is given by:
.
Hence required equation is 
0.1 decrease in 15 minutes
unit rate is 0.1 / 15 = 0.00667
Temperature is decreasing by 0.00667 degrees Celsius per minute