Two fifths is equivalent to forty hundredths, and because all percents are out of a hundred, this can be written as 40%.
Just as with converting fractions to decimals, this technique doesn't always work. The good news is that there are no new steps to learn. Convert the fraction to a decimal, then the decimal to a percent.
Step 1: turn the fraction into a decimal: divide the denominator into the numerator.
Step 2: turn the decimal into a percent: multiply the decimal by 100.