Choice A is the correct answer. Let $s$ = small coffees and $L$ = large coffees.

Step 1: Set up the system: $\begin{cases} s + L = 80 \\ 3s + 5L = 340 \end{cases}$

Step 2: From the first equation: $L = 80 - s$

Step 3: Substitute into the second equation: $3s + 5(80 - s) = 340$$3s + 400 - 5s = 340$$-2s = -60$$s = 30$

Solution: 30 small coffees (and 50 large coffees)

Verification: $30 + 50 = 80$ ✓ and $3(30) + 5(50) = 90 + 250 = 340$ ✓

💡 Strategic Tip: In mixture/combination problems, one equation represents quantity, the other represents value.

Choice B is incorrect because $3(40) + 5(40) = 320 \ eq 340$.

Choice C is incorrect because $3(50) + 5(30) = 300 \ eq 340$.

Choice D is incorrect because $3(20) + 5(60) = 360 \ eq 340$.