You cannot. Forty tenths is, in fact, equal to 4. So the answer is 3 + 4 = 7 which is an integer, not a fraction.
43.2
Forty three and nine tenths.
43.8
Forty nine and three tenths.
Forty and three tenths or forty dollars and thirty cents.
Three hundred forty-six and five tenths.
643.7
3042.2
4.3
Four hundred forty-two and three tenths.
4 3/10
3 5/10.