Unit 6 · Algebra
Age Line → Fred Is 11 Years Older Than Lisa
Lesson 9 of 12
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About this lesson
Four people, three age clues. Express everyone in terms of Lisa: Sam = Lisa+5; Dave = Sam−7 = Lisa−2; Fred = Dave+13 = Lisa+11. Fred − Lisa = 11. The chain shortcut: net difference = +5 − 7 + 13 = +11. Option E (15 = 7+5+13) traps students who just add all numbers.
Chapters
0:00Introduction
0:21The Question
0:54Establishing the Baseline (Dave and Sam)
1:15Positioning Lisa on the Age Axis
1:38Adding Fred and Calculating the Target Gap
2:14Final Option Verification
2:37Correct answer
Question
Dave is 7 years younger than Sam.
Sam is 5 years older than Lisa.
Fred is 13 years older than Dave.
Fred is older than Lisa.
How many years older is Fred than Lisa?
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