If bert had 3 cents more he would have twice as much
as georgia. if he had 4 cents less, he would
have the same amount.
how many cents does bert have?

Let x be the number of coins bert had 
let y be the number of coins georgia has 

x+3 =2y equation 1 
x-4 = y equation 2 

now, since y=x-4, you can put that in the first equation 

it becomes x+3= 2(x-4) right because y is the same thing as x but you can't solve for 2 letters so we have to make them the same. 

so your new equation is x+3 = 2(x-4) 

expand so you get x+3= 2x -8 
then solve for x by putting the x's on one side and the numbers on the other 

bring x over --- 3= 2x- 8 - x and subtract the xs to get 3= x-8 

then bring the 8 to the other side 
3+ 8 = x 
x= 11 
note: remember! when you're bringing numbers and letters to the other side, you have to reverse the operation: say you have x+1 = 2 --- when bringing the one over to the other side it becomes a -1 x+1= 2-1 

so bert has 11 cents. 
this question is a little difficult, it takes some practice and it's stuff you'll learn in high school so don't worry too much if you don't get it 

The density of the cube is 40