Mathematical Puzzles and Problems: Numbers and Logic

Riddle 8:
I am a number. Add one to me, and you get the same result as multiplying me by three. What number am I?

Solution 8:
The number is 2.
2 + 1 = 3 (which is the same as 2 × 3)

Riddle 9:
I am a prime number. When reversed, my digits create another prime number. What number am I?

Solution 9:
The number is 13.
13 is prime, and when reversed, you get 31, which is also prime.

Problem 6:
Simplify the expression: 4(2x + 3) – 2(3x – 5).