Hi! I'm a lurker, who has decided to start posting because I need desperate help!

I took the Official GMAT Practice test and got some DS and PS questions wrong, but I can't figure out why. Can someone please help? Below are the questions and the answers I thought were accurate (OE also specified)

1) The perimeter of an isosceles triangle is 16 + 16\sqrt{2} What is the length of the hypotenuse?

a) 8

b) 16

c) 4\sqrt{2}

d) 8\sqrt{2}

e) 16\sqrt{2}

OE is B but I can't understand how?

2) Value of 10^8 - 10^2/10^7-10^3 is closest to which of the following:

a) 1

b) 10

c) 10^2

d) 10^3

e) 10^4

I simplified it to read 10^6/10^4 = C, but OE is B and I don't know where I'm messing up.

3) When a tree was first planted, it was 4ft tall, and the height of the tree increased by a constant amount each year for six years. At the end of the sixth year, the tree was 1/5th taller than it was at the end of the 4th year. By how many ft. did the tree increase each year?

a) 3/10

b) 2/5

c) 1/2

d) 2/3

e) 6/5

I just guessed 2/3, but I cant form an accurate equation to solve it. I selected x as the constant, so the most I could get was: 4 + 5x = 4 + 3x + 1/5 (4+3x)

I am also unconvinced about some of the answers to the DS questions, which I will post in the appropriate forum.