Linear interpolation:

1. Assume that the increase between 60 and 80 or 4000 and 5000 respectively is proprtional. That means: To reach the 67 you have to calculate: . So the increase is of the difference of the 2 corresponding values.

Now calculate the 2 values in the 4000 and 5000 column.

2. The increase in the last row is of the difference between the 2 corresponding values in the last row.

3. Now calculate the increase between the newly calculated values.

4. I've attached a table with the results.