Bob Phillips wrote...
or even

=AVERAGE(IF(OR(LEFT(A2:A640,2)="TU"),(LEFT(A2:A64 0,5)="ANA36")),B2:B640)

array entered.
....

Testing is good. Testing is your friend. Especially when you're still
up way too late.

Does your OR(..) term return an array? If not, it doesn't do the same
thing that Peo's formula does.