Why is 2.3 - 20 = 17.7 reasonable

If 20-2.3=17.7, then if you add 17.7 to 2.3. it equals 20 which is called the communicative property because whatever you add or subtract, the answer is the same
17.7+2.3=2.3+17.7
the correct answer to this is that 2.3 -20=-17.7 the answer is resonable because if you go on a number line then you would see that if you start a 2.3 and subtract 20 spaces you get to 17.7

You have the problem backwards. It is 20 - 2.3= 17.7. This is because 17.7 plus 2.3 equals 20.