I've been really rusty on math. So excuse me if I sound dumb. I am trying to find equation for the data below:
x y
67 0.055
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85 0.0275
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103 0.01375
What would be the approach to solve this? I am trying to find the values for y.
EDIT: the x values are temp in °F and the y value @ 85 is calculated. The top and bottom values for y are double and half the value of y @ 85.
If there is a systematic reason that the $y$ value is cutting in half every $18$ degrees, then this relationship is exponential. (You can also check out the wikipedia article on half-life.) In this case you can model the relationship by the equation
$$y = 0.0275 \times 0.5^{(x-85)/18}$$
(Alternatively, $y = 0.055 \times 0.5^{(x-67)/18}$ or $y = 0.01375 \times 0.5^{(x-103)/18}$; they are all equivalent.)
The logic is that, since your $y$ is multiplying by $0.5$ every time $x$ increases by $18$ degrees, it must be multiplying by $0.5^{1/18}$ every time $x$ increases by a single degree, so that after $18$ degrees it has multiplied by $(0.5^{1/18})^{18} = 0.5$.
In my first equation, I picked one of the values you gave, $0.0275$, which was the value at $x=85$; then if $x$ is more than $85$, $x-85$ is the number of degrees beyond $85$, so $0.0275$ will have to be multiplied by $0.5^{1/18}$ $x-85$ times, i.e. multiplied by $(0.5^{1/18})^{x-85} = 0.5^{(x-85)/18}$. This is where the formula came from. Similar logic works if $x$ is less than $85$. And the other two equations I gave come from applying the same logic to the other two known points.