Extrapolation is the process by which a value is estimated by using a previously known set of numbers. Interpolation is where a value is estimated between two known values. In this case, if you wanted to predict the value of the y-variable when the x-variable is 15, you would be extrapolating beyond the known data-set.
In math we can define extrapolation as the process of estimating a value that is beyond our range of observation and if we look at the graph we can observe that the variable only has matcihng values with up until . We want to predict the value of the variable when is and this is outside the range of observation.
