Question 87·Hard·Systems of Two Linear Equations in Two Variables
One of the two equations in a system of linear equations is
The system has no solution. Which equation could be the second equation in this system?
For systems of two linear equations, quickly recall that no solution means the lines are parallel but distinct. Put both equations into a comparable form (often by clearing fractions and rewriting in standard form), then compare the ratios of the and coefficients and the constants: if the and coefficient ratios match but the constant ratio does not, the system has no solution. This ratio check is usually much faster on the SAT than actually solving the system.
Hints
Think about line relationships
For two linear equations to have no solution, what must be true about the relationship between their lines on a graph?
Clear the fraction first
Try multiplying the entire given equation by 2 so you can compare it more easily to the answer choices.
Compare coefficients and constants
After rewriting the first equation, compare the coefficients of and and the constant term to each answer choice. Which choice has and coefficients in the same ratio as the first equation but a different constant term?
Desmos Guide
Graph the given equation
In Desmos, type the given equation as 3/2x + 5y = 20 (or (3/2)x + 5y = 20). This plots the first line.
Test each answer choice one by one
Type each answer choice as a separate equation, for example 3x + 10y = 40, then 3x + 10y = 30, then the others. For each one, see how its line relates to the original line.
Decide which relationship gives no solution
In Desmos:
- If the new line overlaps exactly with the original, the system has infinitely many solutions.
- If the new line crosses the original at exactly one point, the system has one solution.
- If the new line is parallel and never meets the original, the system has no solution.
Identify the choice whose line is parallel to the original and never intersects it; that is the one that makes the system have no solution.
Step-by-step Explanation
Recall when a system has no solution
For two linear equations in and :
- One solution: lines intersect in exactly one point (different slopes).
- Infinitely many solutions: lines are the same (all coefficients and constants are proportional).
- No solution: lines are parallel but different (the and coefficients are proportional, but the constants are not).
Rewrite the given equation in a simpler form
The given equation is
To clear the fraction, multiply every term by 2:
So the first line can be written as .
Determine what the second equation must look like
For the system to have no solution, the second equation must represent a line that is parallel to but different from .
That means:
- The coefficients of and must be in the same ratio as and (so the line has the same slope).
- The constant term must be different from 40 (so it is not the same line).
Look for an answer choice where the left-hand side matches (or is a constant multiple of it), but the number on the right side is not .
Match with the correct answer choice
Among the choices, has the same and coefficients as but a different constant term.
So this second equation is parallel to the first line but not the same line, which makes the system have no solution.
Therefore, the correct answer is . (Choice B)