Approach: Stock A is fully known, so peel its value off the total first. What remains is a simple two-variable system for B and C, where the share-count constraint lets you eliminate one variable instantly.
Step 1: Remove stock A. Value of A held: \[ 10 \times 120 = 1200. \] Remaining value in B and C: \[ 3300 - 1200 = 2100. \]
Step 2: Set up B and C. Let B-shares \(= x\), C-shares \(= y\). We are told \[ x + y = 20, \qquad 90x + 150y = 2100. \]
Step 3: Eliminate \(y\). Substitute \(y = 20 - x\): \[ 90x + 150(20 - x) = 2100 \implies 90x + 3000 - 150x = 2100 \implies -60x = -900 \implies x = 15. \]
Step 4: Sanity check. Then \(y = 5\): value \(= 90(15) + 150(5) = 1350 + 750 = 2100\), and with A's \(1200\) the total is \(3300\). Correct.
\[ \boxed{x = 15 \text{ shares of B}} \]