When the interest rates in bonds is in question what would interpolation could recommend as a relevant solution?

INTERPOLATE :-

(Mathematics.) to find or insert (an unknown term) between two known terms in a series.

In 2007 the bonds were issued @ 4.5

In 2009 the bonds were issued @ 5.0

One can suggest that the bonds issued in 2008 were issued at approximately 4.75.