Interpolation

Interpolation uses a linear model to estimate the value of a variable on an interval.
Suppose variables  
\[A, \;\]
  are measured and take the values  
\[(A,B)=(20,35), \; (30, 60)\]
.
It is required to find the value of  
\[B\]
  at  
\[A=23\]
. Using the linear model
\[\frac{B-B_1}{A-A_1}=\frac{B_2-B_1}{A_2-A_1}\]

gives  
\[\frac{B-35}{23-20}=\frac{60-35}{30-20} \rightarrow B= \frac{60-35}{30-20} \times (23-20)+35=42.5\]