Great Work and Rate problem — this one tests a concept I call "decoding relative time statements before setting up equations."
Key concept: Work Rate problems with relative time comparisons (Problem Solving).
Step 1 — Assign variables based on given relationships.
Let Jisoo's time = J hours.
- Minho takes 1 hour longer than Jisoo → M = J + 1
- Minho takes one-third as long as Taeyang → M = T/3, so T = 3M = 3(J + 1)
Step 2 — Set up the combined pair times.
When two people work together, combined time = (A × B)/(A + B).
- Minho + Taeyang time = (J+1) × 3(J+1) / ((J+1) + 3(J+1)) = 3(J+1)2/4(J+1) = 3(J+1)/4
- Taeyang + Jisoo time = 3(J+1) × J / (3(J+1) + J) = 3J(J+1)/(4J+3)
Step 3 — Apply the "75% longer" condition.
"75% longer" means M+T pair time = 1.75 × T+J pair time:
3(J+1)/4 = 1.75 × 3J(J+1)/(4J+3)
Divide both sides by (J+1) [valid since J > 0]:
3/4 = 5.25J/(4J+3)
3(4J+3) = 21J
12J + 9 = 21J
J = 1 hour
Step 4 — Find individual times.
Jisoo = 1 hour (60 min), Minho = 2 hours (120 min), Taeyang = 3 × 2 = 6 hours (360 min)
Step 5 — Calculate combined rate for all three.
Combined rate = 1/1 + 1/2 + 1/6 = 6/6 + 3/6 + 1/6 = 10/6 per hour
Combined time = 6/10 hours = 0.6 hours = 36 minutes
Answer: B (36 minutes)
Common trap: Students read "Minho takes one-third as long as Taeyang" and write T = 3J (mistakenly linking to Jisoo rather than Minho). Always track exactly who each comparison refers to. Here, both comparisons are about Minho: 1 hour longer than Jisoo, one-third as long as Taeyang.
Takeaway: In multi-person rate problems, express every individual time in terms of one variable first — then build combined-pair equations. Trying to work with multiple variables simultaneously is where most errors sneak in.