What is 0.4 into a percentage?

.4 is 4/10 or 40/100 so it is 40% If you want, think .1 as 10% and .4 is 4x10% or 40%. To convert any number into percent, just make it into a fraction with the bottom, the denominator, equal to 100. The top, the numerator, is the percent. So in your case, .4 needs to be made into a fraction with 100 on the bottom. but we know that .4 is 4/10 and if we multiply the top and bottom by 10 we get 40/100