Question 104·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 two linear equations on the SAT, quickly decide between elimination and substitution by looking at the coefficients: if one variable has coefficients that are already opposites or easy to make opposites (like and here), use elimination to cancel that variable, solve for the remaining variable, then substitute back to find the other. Always check that your final pair satisfies both original equations to avoid small arithmetic or sign mistakes.
Hints
Choose a method
You want the value of from a system of two equations. Would it be easier to first eliminate one variable (elimination) or solve one equation for a variable and substitute (substitution)?
Make coefficients cancel
Look at the -terms: one equation has and the other has . What could you multiply the second equation by so that its -coefficient becomes and cancels with when you add the equations?
Find first, then
After you eliminate , you will get an equation with only . Solve that for , then plug that value into either original equation to solve for .
Check your solution
Once you find a candidate value for , substitute it (with the corresponding ) back into both original equations to make sure they are both true.
Desmos Guide
Enter the first equation
Type y = (20 - 3x)/2 into Desmos. This is the same as , solved for .
Enter the second equation
On a new line, type y = 5x - 3, which is the second equation solved for .
Find the intersection point
Look for the point where the two lines intersect and tap or click on it; note the -coordinate of that intersection. That -value is the answer to the question.
Step-by-step Explanation
Identify the goal and a method
We are given a system of two equations:
We want the value of in the solution . A fast way is elimination: make the coefficients of opposites so they cancel when we add the equations.
Eliminate one variable (elimination method)
Multiply the second equation by so that its -term becomes :
Second equation:
Multiply by :
Now line this up with the first equation and add them:
When you add these, the and cancel.
Solve for
Add the two equations:
So we have the -value of the solution. Next we use this to find .
Substitute back to find
Use either original equation. The second one is simpler:
Substitute :
So the solution has , which corresponds to answer choice C.