Percentage of an amount? I'm totally confused, we were doing a question in class and there are two answers but I'm not sure why one works and the other one doesn't. 
For example; there are 6000 pandas now and over 10 years they have decreased by 40%. How many pandas were there 10 years ago? There are two methods that I have found.
Method 1. 10% of 6000 is 600 x 4 = 2400 + 6000 = 8400 pandas.
Method 2. 6000 = 60% devide by 3 = 2000 x 5 = 10000 pandas.
Then though 40% of 8400 is 3360 which if you take off 8400 isn't 6000.
But if you use method two it works.
Why is this? I mean how come one method works and the other doesn't?
I know this is pretty easy but I just really want to know, thanks. 

In Method 1, you calculated $40\%$ of the number of pandas NOW. Since those 6000 pandas represent 60% of the original, you calculated $40 \%$ of $60\%$, that is: 24% of the original amount.
In method $2$, you correctly calculated what the original number of pandas must be if $60\%$ of that original number reduces the number to $6000$ now existing pandas. 
In short, we're given that $6000$ is the number of pandas that remain after a $40\%$ decrease in the original number of pandas $(x)$ ten years ago. In math, we can write this as
Let $x = \text{the number of pandas 10 years ago}$. We know that $40\% x = \dfrac{40}{100} x = 0.4x$.
$$x - 0.4 \cdot x = 6000 \iff 0.6 x = 6000$$
Solving for $x$ gives us that there were $x = \dfrac {6000}{0.6} = 10,000 \,\text{ pandas 10 years ago}$.
