Answer: D

Multiply both sides of the equation by $12$, the least common denominator: $12 \cdot \frac{x}{3} + 12 \cdot \frac{y}{4} = 12 \cdot 5$. This gives $4x + 3y = 60$.

Choice C multiplies $4$ and $3$ each by $5$ and adds, but $4x + 3y$ is $12$ times the left side of the equation, so it equals $12 \times 5$.

Why the other choices are wrong

• A. 5 is the value of x/3 + y/4. The expression 4x + 3y is 12 times that sum.
• B. 12 is the number to multiply the equation by, not the value of 4x + 3y.
• C. This adds 4 × 5 and 3 × 5. Multiplying the whole equation by 12 shows that 4x + 3y = 12 × 5.