Ben is 20 years older than Daniel. Ben and Daniel first met two years ago. Three years ago, Ben was 3 times as old as Daniel. How old is Daniel now?

The first thing to do is identify the unnecessary information (that they met 2 years ago).  

From there, let's go back three years ago.  We'll call Daniel's age x, and Ben's age will be 3x.  Ben's age is also equal to x + 20.

3x = x+ 20

2x = 20

x = 10

Daniel was 10 three years ago, and Ben was 30.  Now, Ben is 33.

4 and 5)16.97 or 12√2

6)15.59 or 9√3

7)y = 15.59 or 9√3     x =27

8)17.80