Choice B is the correct answer. Let $n$ = notebooks and $p$ = pens.

System: $\begin{cases} n + p = 25 \\ 3n + 2p = 65 \end{cases}$

Step 1: From first: $p = 25 - n$

Step 2: Substitute: $3n + 2(25 - n) = 65$$3n + 50 - 2n = 65$$n = 15$

Solution: 15 notebooks (and 10 pens)

Verification: $15 + 10 = 25$ ✓ and $3(15) + 2(10) = 45 + 20 = 65$ ✓

💡 Strategic Tip: One equation for quantity, one for total cost.

**Other choices fail verification.