Lookup Data In Another Workbook & Return Corrrespoding Value

I have two excel workbooks, i.e target.xls and source.xls. Assume

target.xls looks like this

letter number sound
a 1 a
b 2 be
c 3 cee
d 4 dee
e 5 ee
f 6 eff
g 7 gee

source.xls looks like this

letter number
a I
c III
d IV
g VII

In the target file, I want to be able to change the numbers of the letters found in the source file, so that the new target file will look like this.
new target.xls

letter number sound
a I a
b 2 be
c III cee
d IV dee
e 5 ee
f 6 eff
g VII gee

I believe there must be a simple way to do this using VBA.