Fractions, decimals and percents: which way?
Each direction takes one step. To go from a fraction to a decimal, divide the top by the bottom. To go from a decimal to a percent, multiply by 100. To go from a percent to a fraction, write it over 100 and simplify.
A worked example
3/5 means 3 ÷ 5 = 0.6. Then 0.6 × 100 = 60%, and 60% is 60/100. Divide the top and the bottom by 20 to get 3/5. 0.75 is 75 hundredths, or 75/100, which is 3/4. Recap: the three forms name one amount, and each arrow is one step.
The mistake people actually make
Percent means out of 100, so 0.4 is 40%, not 0.4%. When you simplify, divide the top and the bottom by the same number.