How do you work out 21.3-8= show your working. plz

21.3-8
You would line up the decimals. In this case, 8 would turn into 8.0 because it is a whole number. *However many places to the right the number with decimals goes, you add in the zeros for the other number(s) that do not have decimals in them already. Make sense?
Then you would just subtract.
21.3
- 8.0
13.3 = your answer
How did I get this answer exactly? Well, 3-0=3. 1 is less than 8, so borrow a 1 from the next column to make 11. 11-8=3. Because you borrowed 1 from 2, it is now just the number 1. 1-0=1
Hope I helped!