## Using steam tables

Well, im studying for my engineering thermodynamics exam, and i dont seem to have written down some of the information about steam tables.

Im fairly confident using them, except i cant remember the exact formulas we use to get the numbers in between the numbers given. For example, i have a question as follows:

Use Rogers and Mayhew to determine specific internal energy change for water from 17.5 degrees Celsius to 86 degrees Celsius.

Well, my problem is the steam tables only give temperatures of 17 and 18, and 85 and 90.
17 bar - Hf 71.3
18 bar - Hf 75.5
85 bar - Hf 355.9
90 bar - Hf 376.9

how can I use this information to get the Hf of water at 17.5 degrees Celcius, and 86 degrees Celcius??