Question 125·Medium·Systems of Two Linear Equations in Two Variables
The solution to the system of equations is . What is the value of ?
For systems of linear equations on the SAT, always check the coefficients first to see if elimination is easy. If one variable has coefficients that are already opposites (like and here), add or subtract the equations to eliminate that variable in a single step, then solve the resulting one-variable equation. If the test only asks for one variable (just or just ), stop as soon as you find that value instead of solving for both, to save time.
Hints
Look at the coefficients of y
Compare the terms in both equations. How are and related?
Try combining the equations
What happens to the terms if you add the two equations together, left side with left side and right side with right side?
Use the new one-variable equation
After you combine the equations and disappears, you will have an equation with only . Solve that equation carefully and match your result to one of the answer choices.
Desmos Guide
Enter the equations as lines
In Desmos, type the first equation as 4x - 7y = 8 and the second equation as 3x + 7y = 27. Desmos will graph both lines.
Find the intersection point
On the graph, locate the point where the two lines intersect. Click or tap on that intersection point to display its coordinates .
Read the x-value
Look at the x-coordinate of the intersection point shown by Desmos; that x-value is the solution to the system and matches one of the answer choices.
Step-by-step Explanation
Notice how to eliminate a variable
Look at the -coefficients in the two equations:
- First equation: (coefficient of is )
- Second equation: (coefficient of is )
Because these coefficients are opposites, adding the two equations will make the terms cancel.
Add the equations to remove y
Add the left sides and the right sides of the equations:
So the combined equation simplifies to .
Solve for x and pick the matching choice
From , divide both sides by :
So the value of is , which corresponds to answer choice C.