In this case, `(x_1, y_1) = (1,143.5)` and `(x_2, y_2) = (2, 137.9)`.

Using the method of proportions:

`b = y_1 + (y_2 - y_1)(a - x_1)/(x_2 - x_1)`

` = 143.5 + (137.9-143.5)(1.4-1.0)/(2.0-1.0)`

`= 143.5 + (-5.6)0.4/1.0`

` = 143.5 - 2.24`

` = 141.3^@\ "C"`

This is the estimate of the temperature after `1.4\ "min"` using linear interpolation.

Easy to understand math videos:
MathTutorDVD.com