Choice B is the correct answer. Let $b$ = boat speed in still water and $c$ = current speed.

Analysis:

• Downstream speed = $b + c$ (current helps)
• Upstream speed = $b - c$ (current opposes)
• Distance = Speed × Time

Step 1: Set up the system: $\begin{cases} (b + c) \cdot 3 = 48 \\ (b - c) \cdot 4 = 48 \end{cases}$

Step 2: Simplify: $\begin{cases} b + c = 16 \\ b - c = 12 \end{cases}$

Step 3: Add the equations: $2b = 28$$b = 14$

Step 4: Find $c$: $14 + c = 16$$c = 2$

Solution: Current speed is 2 mph

Verification: Downstream: $(14 + 2) \times 3 = 48$ ✓, Upstream: $(14 - 2) \times 4 = 48$ ✓

💡 Strategic Tip: Current problems: downstream = boat + current, upstream = boat - current.

Choice A is incorrect because with $c = 1$: upstream speed would be $(15) \times 4 = 60 \ eq 48$.

Choice C is incorrect because with $c = 3$: downstream speed would be $(13) \times 3 = 39 \ eq 48$.

Choice D is incorrect because with $c = 4$: upstream speed would be $(12) \times 4 = 48$ ✓ but downstream would be $(16) \times 3 = 48$.