Hilda has $210 worth of $10 and $12 stock shares. The number of $10 shares is five more than twice the number of $12 shares. How many of each type of share does she have?

Let's first define variables.
 y: number of shares of $ 10
 x: number of shares of $ 12
 We write the system of equations:
 10x + 12y = 210
 y = 2x + 5
 Solving the system:
 x = 75/17
 y = 235/17
 Answer: 
 she has: 
 75/17 shares of $ 12 
 235/17 shares of $ 10