Then you assume that there is a linear correlation between those points and so you can derive the value of mid point.
You have a table with median sale prices of single family homes sold for the years from 1975 to 1995.
In this case you have to use the median sale price of single family homes sold at two years: 1994 and 1995.
You assume that the prices of the table are for the end of the year, then the price sold in June 1995 is right in the middle of the sold price in1994 and 1995.
So, the interpolation will give to [price in 1994 + price 1995] / 2 = [140,400 + 141,400] / 2 = 140,900
