Probability basics

If i have to chose two specific numbers from a set of 5 numbers:
Eg: 2 and 3 from set {1,2,3,4,5}

I understand the basic counting principle gives 2/5. (2 out of 5)
Using Combinations Will it be 2c2/5c2? or 2c2/5p2?

What about ?
0.5% of x + 0.8% of y= 0.75% of (x+y)
solving we get y= 5x.
hence y proportion is 5/6

Probability

If there are 10 balls, 3 red, 5 blue and 2 green. I pick two without replacement, what is the probability that both are red?

I find two ways of doing this, clearly one is wrong!

1) 3/10 * 2/9

2) (3C1 *2C1)/10C2

Can someone please help me understand this!

Averages

A certain mixture of nuts consists of 4 parts almonds to 1 part walnuts, by weight. By removing some of the almonds, the remained nuts consist 75 percent of almonds, by weight. What percent of the almonds were removed?
(A) 10%
(B) 12.5%
(C) 20%
(D) 25%
(E) 30%