Get averages by category

I'm trying to find the average velocity of particles. The program gives instantaneous velocities for the particles but not the average. I'm left with an excel file with thousands of instant velocities for dozens of particles. There are varying amounts of values per particle, some have maybe 7 values, others might have 30.

I can manually select which values to average, but I'd like to know if I could have excel do that automatically.

For instance, in collum B I'd have the values I want to get averages from, in A I have which category they belong to. Rows 1-7 might be particle A, rows 8-20 might be particle B.

Is there a way for excel to get the averages in B (velocity) for each category (particle) in A?

I did consider that, but I would need to setup one for each category since I want the averages of all particles. The data is sorted by particle number, so that wouldn't save me any effort: selecting the data for each particle and averaging that is the same amount of work as saying "average if particle one, average if particle two..."