10 tenths = 1 one. Regroup when the tenths sum to 10 or more.

Tenths	=	5 tenths + 8 tenths = 13 tenths
Regroup	=	13 tenths = 1 one + 3 tenths
Ones	=	5 + 2 + 1 (carry) = 8
Sum	=	8.3