Narrowing a table into an area

[Hall]

My table looks like this:

A 100
A 110
B 300
B 310
B 320
C 200
D 210
D 220

How would I identify the rows with B in the first column to be used in the
INDEX formula's first parameter?

=INDEX( [rows with B], 2, 2 )
giving me the value 310

Thanks!

 

[KC Rippstein]

This will give you the answer you want...you are specifically telling the
formula to find the second value (represented by +1) in column B that has a
value of "B" in sorted column A:
=INDEX(B:B,MATCH("B",A:A,FALSE)+1)