It can be helpful to remember that "%" is just a shorthand way to write "/100" (divided by 100, or per 100, or per cent). ("Cent" is often used to refer to 100 or 1/100. A "cent" is 1/100 of a dollar, or 1/100 of an acre, for example. A century is 100 years.)
Make the denominator of your fraction be 100. Multiply numerator and denominator by 20/20.
(2/5)×(20/20) = 40/100 = 40%
You can also get there via decimals.
... 2/5 = 0.4 = 0.40 (40 hundredths) = 40/100 (40 hundredths) = 40%
Or, you can multiply by 100%.
... (2/5)×(100%) = (200/5)% = 40%