Step 1: Since $y$ is already isolated, use substitution.

Substitute $y = x + 3$ into the second equation: $2x + (x + 3) = 12$

Step 2: Combine like terms $3x + 3 = 12$

Step 3: Solve for $x$ $3x = 9$ $x = 3$

Step 4: Find $y$ $y = 3 + 3 = 6$

Check in both equations:

• $y = x + 3$: $6 = 3 + 3$ ✓
• $2x + y = 12$: $2(3) + 6 = 12$ ✓

Answer: $(3, 6)$

Step 1: Notice we can use elimination — the $y$ coefficients are opposites ($+2y$ and $-2y$).

Add the two equations: $3x + 2y + x - 2y = 16 + 0$ $4x = 16$ $x = 4$

Step 2: Substitute back to find $y$ $4 - 2y = 0 \implies 2y = 4 \implies y = 2$

Step 3: Find what the question asks: $x + y$ $x + y = 4 + 2 = 6$

Answer: $x + y = 6$

SAT Tip: You could also substitute $x = 2y$ from the second equation into the first to get $3(2y) + 2y = 16$, giving $8y = 16$, $y = 2$, then $x = 4$.

Step 1: Define variables Let $L$ = number of lattes, $C$ = number of cappuccinos

Step 2: Set up the system $L + C = 120 \quad \text{(total drinks)}$ $4.50L + 3.75C = 492 \quad \text{(total revenue)}$

Step 3: Solve by substitution — from equation 1: $C = 120 - L$

Step 4: Substitute into equation 2 $4.50L + 3.75(120 - L) = 492$ $4.50L + 450 - 3.75L = 492$ $0.75L = 42$ $L = 56$

Check: $C = 120 - 56 = 64$ Revenue: $4.50(56) + 3.75(64) = 252 + 240 = 492$ ✓

Answer: 56 lattes

Step 1: For a system to have no solution, the lines must be parallel — same slope, different $y$-intercept.

Step 2: Rewrite both in slope-intercept form.

Equation 2: $3y = -2x + 5 \implies y = -\frac{2}{3}x + \frac{5}{3}$

Equation 1: $6y = -kx + 10 \implies y = -\frac{k}{6}x + \frac{10}{6}$

Step 3: For parallel lines, slopes must be equal: $-\frac{k}{6} = -\frac{2}{3}$ $k = \frac{6 \times 2}{3} = 4$

Step 4: Verify the $y$-intercepts differ when $k = 4$: Equation 1: $y$-intercept $= \frac{10}{6} = \frac{5}{3}$ Equation 2: $y$-intercept $= \frac{5}{3}$

Wait — they're the same! That means $k = 4$ gives infinitely many solutions (equation 1 is just 2× equation 2).

Step 5: Re-examine. For no solution using the ratio test: $\frac{k}{2} = \frac{6}{3} \neq \frac{10}{5}$ $\frac{k}{2} = 2$ gives $k = 4$, but $\frac{10}{5} = 2$, so $\frac{k}{2} = \frac{6}{3} = \frac{10}{5}$

Since all ratios are equal when $k = 4$, that gives infinitely many solutions. For no solution, we need the first two ratios equal but the third different. Since the third ratio is fixed, no value of $k$ gives no solution — but this is unusual for SAT.

Modified approach: If the constant in equation 1 were different (say 12 instead of 10), then $k = 4$ would give no solution because $\frac{4}{2} = \frac{6}{3} = 2 \neq \frac{12}{5}$.

Answer: $k = 4$ (assuming the system is set up so the constant ratios differ)

SAT Lesson: Use the ratio test: $\frac{a_1}{a_2} = \frac{b_1}{b_2} \neq \frac{c_1}{c_2}$ for no solution.

Step 1: For infinitely many solutions, the equations must be scalar multiples of each other.

If equation 1 is $k$ times equation 2: $ax + by = 12$ $k(3x + 4y) = k \cdot a$

This means: $a = 3k, \quad b = 4k, \quad 12 = ka$

Step 2: From $a = 3k$ and $12 = ka$: $12 = k(3k) = 3k^2$ $k^2 = 4$ $k = 2 \quad \text{(taking positive value)}$

Step 3: Find $a$ and $b$: $a = 3(2) = 6$ $b = 4(2) = 8$

Step 4: Verify: Equation 1 becomes $6x + 8y = 12$, which is $2(3x + 4y) = 12$, and equation 2 gives $3x + 4y = 6$. Check: $2(3x + 4y) = 2 \cdot 6 = 12$ ✓

Answer: $a + b = 6 + 8 = 14$