I am having trouble with decimals. If i add 8+3+0.9+0.31=51 am i correct

Answer 1

No. You are not correct.

You only have two whole numbers. They add up to 11.

You have two numbers less than one.

. 0.90
+ 0.31
_______
= 1.21

11 + 1.21 = ?

Think of money.

8 dollars + 3 dollars + 90 cents + 31 cents.

Answer 2

the decimal places must be on the same line when adding.
so it should look like this
.9
.31
8.0
+3.0
______

i hope that helped..!