Basically something like this;
(Averaging a range on Sheet 7 and adding it to the Average of a range on Sheet 12, then average the two together...)
=AVERAGEIFS(Sheet7!$Q$14:$T$14,Sheet7!$Q$14:$T$14,">5",Sheet7!$Q$14:$T$14,"<10")+AVERAGEIFS(Sheet12!$Q$12:$T$12,Sheet12!$Q$12:$T$12,">15",Sheet12!$Q$12:$T$12,"<20")/2