For some reason, Excel does not have a built in linear interpolation function. Quattro Pro has one, and other spreadsheets might, but Excel doesn't.

If you are allowed to use UDF's, some have offered UDF's for linear interpolation (user shg comes quickly to mind). You might be able to find those offerings through this site's search engine.

If you would not be allowed to use something downloaded from the internet, it is easy enough to build linear interpolation into a spreadsheet if you break the process down into steps.

1) Locate the interval. The =MATCH() function works well
2) Return the x and y values bracketing the desired value. The INDEX() function works well, using the results obtained from the MATCH() function.
3) Calculate interpolated unknown. The =TREND() function can be used here, or you can use an algebraic expression for a straight line.