Rule of four ? Calculate number given other 5

Question

I have a pair of values x y ( 4 numbers, not 3 ) e.g.
x -> y
--------
29 -> 20
15 -> 100

How can I calculate a middle value e.g.
21 -> ?
given the min and max values of each columns x and y

what I'm looking is for the sequence but only knowing 2 pairs

following this example, the list of number could be
23 -> approx 40
21 -> approx 60
18 -> approx 80
etc

is possible to extrapolate the sequence ?

thanks!!

Was solved by Ross!, thanks a lot

29 -> 20
28 -> 25.714285714286
27 -> 31.428571428571
26 -> 37.142857142857
25 -> 42.857142857143
24 -> 48.571428571429
23 -> 54.285714285714
22 -> 60
21 -> 65.714285714286
20 -> 71.428571428571
19 -> 77.142857142857
18 -> 82.857142857143
17 -> 88.571428571429
16 -> 94.285714285714
15 -> 100

Answer 1

You can certainly view your data as two points on a line and use the two-point form for a line to compute any other points. That is $$y-100=\frac {20-100}{29-15}(x-15)$$ Plugging in $x=21$ gives $y=65\frac 57$

Answer 2

29 -> 20 15 -> 100

y=mx+b

m=(y2-y1)/(x2-x1)

m=(100-20)/(15-29)=80/-14=-5.714...

if x = 0 what is y?

y=(80/-14)*x+b

y=100

x=15

100=(80/-14)*15+b

b=100/((80/-14)*15)=-1.166

y=(80/-14)*x-1.166

https://www.mathplanet.com/education/algebra-1/formulating-linear-equations/writing-linear-equations-using-the-slope-intercept-form