well think of it this way
which is bigger 21 or 40?
40 correct
this rule applies to decimals as well, as long as they are not a negative integer
this number is not negative, and therefore, the number that looks bigger actually is bigger
and we know that 4 is bigger than 2, and since there both are written in the hundredth place of the decimal, we can safely conclude that 0.04 is bigger than 0.021