First, we want to turn the fraction into a decimal
there are two ways of doing this

either treating the fraction as division, since this is what a fraction represents 2 / 5 = 2 : 5 = 0.4

Percentage

or manipulating the fraction to have a denominator consisting of multiples of 10 2 / 5 = 4 / 10 = 0.4

To get rid of a percentage sign, we divide by 100,
therefore,
to add the %, we multiply by 100

0.4(100) = 40%