Question

HELP PLS is 8.4 or 8.40 bigger?

Answer 1

They are the same number.

8.4 is 8 units and 4 tenths.

8.40 is 8 units, 4 tenths and zero hundreths.

So, you've actually added nothing! Indeed, you can add as many zeroes as you like at the end of the decimal expansion:

$8.4 = 8.40 = 8.400 = 8.40000 = \ldots$