I'm trying to find a formula that will calculate the average number of days
it takes for a certain type of material to move through my department. Where
column A is the type of material on a book truck and column I is the number
of days it took for that book truck to move through the department.

If the cells in column A= MUSIC then average the cells of the same row in
column I.

I've been playing with the SUMIF and AVERAGE functions but haven't been able
to figure it out.

It didn't like =AVERAGE(IF(A2:A399,"MUSIC",I2:I399)) as an array formula.

Any help is greatly appreciated.