The arithmetic mean of any amount of equal-sized number groups together is the arithmetic mean of the arithmetic means of each group.
ðɪ ɚɹˈɪθmətˌɪk mˈiːn ʌv ˌɛni ɐmˈaʊnt ʌv ˈiːkwəlsˈaɪzd nˈʌmbɚ ɡɹˈuːps təɡˌɛðɚɹ ɪz ðɪ ɚɹˈɪθmətˌɪk mˈiːn ʌvðɪ ɚɹˈɪθmətˌɪk mˈiːnz ʌv ˈiːtʃ ɡɹˈuːp.