Question

What is 0.4% writen as a decimal, and why thanks :)

Answer 1

0.4 is already written as a decimal because out of 4 out of 10 which is 40 percent.
The 10 is from where the 4 is for example the number 4 on your question is the tenth place so tenth place equals 10 so...
=4/10 

Answer 2

To convert a percentage to a decimal divide by 100: 0.4% = 0.4 ÷ 100 = 0.004

0.04