If you mean to ask how you divide 2 by 5, then consider instead dividing 20 by 5, then divide that result by 10 (which is as simple as moving a decimal point to the left)
We have 20/5 = 4, then 4/10 = 0.4.
Why does this work? All we're doing is multiplying by 1 in the form of 10/10, then moving around factors within the product:
2/5 = 2/5 * 10/10 = (2*10)/5 * 1/10 = 20/5 * 1/10 = 4/10 = 0.4