WAEC 2020 Mathematics · Question 49
Solve (5 - 3x) < (3 - 7x)
Options
- A.x >
- B.x <
- C.x >
- D.x <
Answer: D. x <
Explanation written with AI and checked against the answer key
Step 1: Analyze the question We need to solve a linear inequality and find the range of x. Fractions are involved, so we clear them first.
Step 2: Concept Multiply both sides by the LCM of the denominators to remove fractions. If you multiply or divide by a negative number, the inequality sign flips.
Step 3: Solution
(1/3)(5 − 3x) < (2/5)(3 − 7x)
The LCM of 3 and 5 is 15. Multiply both sides by 15 (positive, so the sign stays):
5(5 − 3x) < 6(3 − 7x)
25 − 15x < 18 − 42x
Add 42x to both sides:
25 + 27x < 18
Subtract 25:
27x < −7
x < −7/27
Step 4: Answer x < −7/27, which matches option D.
Tip: When there are fractions on both sides, multiply by the LCM first. This avoids messy fraction arithmetic. Always check whether you divided by a negative number, because that is the only time the sign flips. Here we divided by +27, so the sign stayed the same.
Quick check: Try x = −1 (which is less than −7/27):
- Left: (1/3)(5 + 3) = 8/3 ≈ 2.67
- Right: (2/5)(3 + 7) = 4
- 2.67 < 4 ✓
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