The number in the three different forms is as follows.
2/5 (fraction), 0.2 (decimal), and 20% (percent).
You could use percentages for the results of a poll. For example, if you held a poll for which flavour of ice cream is your favorite. Let's say you surveyed 100 kids and 50 kids voted chocolate, 30 vanilla, and 20 strawberry. Then you could say 20% voted for strawberry.
You could use the decimal form for money. For example, let's say you want to buy something that costs $10.20 and you only have 10 dollars. You could say that you would need 20 more cents, or $0.20.
As for the fraction, you could use this for measurements. For example, let's say you have a piece of fabric. You want to cut it up into 5 equal pieces, but only use two of those pieces. You could say that you would only use 2/5 of the fabric.
I hope I helped you out! If anything is wrong, please let me know about it! :)
