First u must make this mixed fraction to a fraction by multiplying 8 with 40 which equals to 320 then u add 320 with 21 which equals to 341:
341/40, well now this is an improper fraction. So now u will divide 341 by 40 and whatever u get is ur answer.
341÷40=8.525.
HOPE THIS HELPS AND IS NOT CONFUSING!!!!!! IF IT IS THEN COMMENT BELOW AND I WILL EXPLAIN EASIER.