Answer:
Jackson
0.8×5=4
Xavier
0.8×5=0.4
If Xavier said to Jackson that his answer of 4 can't be right.
Jackson needs a decimal point because a product always have the same answer.
Then,
Xavier is right with his sayings but wrong with the answer of 0.4
The correct answer is 4.0
The decimal point should be counted from the back not from the front as Xavier did