Question 63·Hard·Linear Inequalities in One or Two Variables
During a quality-control test, the moisture content , in percent, of a batch of timber must satisfy both of the following conditions:
- Over-drying guard:
- Flexibility guard:
Which of the following represents all possible values of that meet both conditions?
For SAT questions with a variable that must satisfy two or more linear inequalities, always solve each inequality separately as if it were a normal linear equation problem (distribute, combine like terms, isolate the variable), being careful with inequality directions and with strict (, ) vs. inclusive (, ) signs. Then sketch quick number lines or think in terms of lower and upper bounds: the true solution is the intersection of all ranges (take the largest of all lower bounds and the smallest of all upper bounds). If answer choices are intervals, you can also quickly test one value from each interval in the original inequalities to confirm or eliminate choices.
Hints
Separate the two guards
First, treat each guard (each inequality) on its own. Solve the over-drying guard for , and then solve the flexibility guard for .
Carefully isolate in each inequality
For , distribute the , then move all terms to one side and numbers to the other. For , do the same. Remember: dividing or multiplying by a positive number does not flip the inequality sign.
Combine the solution ranges
Once you have the solution for each inequality, think about the overlap of those ranges on a number line. Which values of make both inequalities true?
Pay attention to strict vs. non-strict signs
Notice that one inequality uses and the other uses . This affects whether the boundary value itself is allowed or not. Keep that in mind when choosing between and in the answer choices.
Desmos Guide
Graph both inequalities in Desmos
In Desmos, use in place of . Enter the two inequalities on separate lines:
-4(2x - 13) + 5 < 3x + 13x + 1 <= 8x - 24
Desmos will shade the regions on the -axis where each inequality is true.
Find the overlapping region on the x-axis
Look along the -axis (this represents ). You should see where the shaded regions from both inequalities overlap. The solution set is the part of the -axis where both inequalities are true at the same time. Note the -value where this overlapping region starts.
Use equalities to get the exact boundary (optional)
To find the boundary more precisely, graph the equations y = -4(2x - 13) + 5 and y = 3x + 1, then use Desmos’s intersection feature to find the -coordinate where they are equal. That -value is the boundary from the over-drying guard. Compare it to to see which requirement is stricter, and then express the final solution as an inequality for .
Step-by-step Explanation
Understand what “must satisfy both” means
The moisture content has to make both inequalities true at the same time:
- Over-drying guard:
- Flexibility guard:
The plan:
- Solve each inequality for .
- Find the intersection (overlap) of the two solution sets.
We will not pick an answer until we know the overlap.
Solve the over-drying guard inequality
Start with:
Distribute the and combine like terms:
Now move all terms to one side and constants to the other. Add to both sides and subtract from both sides:
So, to satisfy the over-drying guard alone, must be large enough that is greater than .
Solve the flexibility guard inequality
Now solve:
Subtract from both sides:
Add to both sides:
Divide both sides by (a positive number, so the inequality direction stays the same):
This can also be written as . So the flexibility guard alone allows any that is at least .
Intersect the two solution sets and express the final answer
From the over-drying guard we have:
Divide both sides by (positive) to solve for :
which is the same as .
From the flexibility guard we have .
Now compare the two lower bounds:
- One requires to be greater than .
- The other requires to be at least .
Since is larger than , the stricter requirement is . Any that satisfies this is automatically .
So the values of that meet both conditions are:
which matches answer choice B.