How do you write 0.4% as a decimal

0.4% as a decimal ISN'T 0.4 or 0.04 because the question is telling us 0.4% not 40% (for 0.4) or 4% (for 0.04) so the answer is 0.004 because the percentage is 0.4% ;)

This is a question that tricks you into thinking there is an obvious answer. You see the decimal in the question and immediately think, thats easy, it 0.4. But 0.4 and 0.4% are entirely different.

In decimals, one whole is symboised by 1. Which is why, if we were to change 0.4 to a percentage, it woukd be 40%, as percentages are out of 100.