What is 0.3% as a fraction?

##3/1000##
A percentage is actually just a part out of 100. So for example, 10% is the same as 10 out of a 100 or ##10/100## which simplifies to ##1/10##.
In this case we see ##0.3%## which is like saying ##0.3/100##. Of course we do not want decimals in the numerator (the same way we wouldn’t want a fraction in the numerator of a fraction). So to fix this we could multiply by ##10/10## (which is to say ##1##) to make ##0.3## into a whole number.
##0.3/100 * 10/10 = 3/1000## and cannot be simplified.