new macbook battery Draining

I just bought my first ever apple product besides an ipod. I bought the new macbook 12 inch. It worked great last night, I turned it on out of the book and used it until it was about to die. I plugged it in to charge and it took 4 hours almost 4 and a half hours to charge to 100%. Then I unplugged it and just looked where everything was on it. Then I saw my battery draining. At 100% it said it had 20 hours which I knew wasn't right and that it would be a lot less. I was told from the guy in the store around 10 hours is what I would get. So I kept looking around and I checked an hour and a half after taking it off the charger and it was down to 10 hours left. I am only getting about 4 maybe 5 hours on it and I have nothing open. I know how to make sure that I quit everything I open. Nothing is open but it dies quickly, is this normal or do I have a bad one. I am worried and just wondering because this is the first laptop I ever bought myself and I am worried I might have made a mistake over this whole battery issue thing And how much it cost. I really loved it until this whole issue with how long it took to charge and the battery not lasting.

Any help would be appreciated.