Stock Portfolio Evaluation

Let the trader hold $x$ shares of B. Then she holds $(20 - x)$ shares of C. Value of Stock A = $10 \times 120 = 1200$. Total portfolio value = $1200 + 90x + 150(20 - x) = 3300$. $1200 + 90x + 3000 - 150x = 3300$ $4200 - 60x = 3300 \implies 60x = 900 \implies x = 15$. Thus, she holds 15 shares of stock B.