linear function

[Su2535]

What is the formula for

if x=5 y=1

if x=10 y=3

if x=15 y=5

if x=12,5 what is the value of y

[#2]

@Su2535 

Not sure that what you've set forth is actually linear.

[#3]

(resubmitted to correct critical typo)

@Su2535 

At the risk of doing your class or exam assignment for you, use LINEST to determine the coefficients of the linear approximation est y = m*x + b.

Formulas:

F3:G5:  =LINEST(B3:B5, A3:A5, TRUE, TRUE)

C3:       =A3*$F$3 + $B$3

Select F3:G5 and type the formula.  In some versions of Excel, we must commit by pressing ctrl+shift+Enter instead of just Enter.

In this case, the linear "approximation" is an exact fit.  This is indicated by r^2 = 1 in F5.

PS....  In general, it is unwise to use LINEST results blindly.  The first step should be to use an XY Scatter chart to visually inspect the relationship between x and y.  You can even create a (linear) trendline to determine the fit.