Key concept being tested: Ratio-based Word Problems with Percentages (Problem Solving)
The smart move on this question is to assign a concrete multiple immediately. Since the factory-to-office ratio is 5:4, let F = 5k and O = 4k.
1. Set up the dental plan totals:
Factory dental: 20% of 5k = k
Office dental: 45% of 4k = 1.8k
Total dental = 2.8k
2. Set up the pension plan totals:
Factory pension: 80% of 5k = 4k
Office pension: 90% of 4k = 3.6k
Total pension = 7.6k
3. Use the given condition — pension exceeds dental by 72:
7.6k - 2.8k = 4.8k = 72
So k = 15
4. Factory minus office employees = 5k - 4k = k = 15
Answer: C (15)
Common trap: Students often set up the equation correctly but then answer "how many factory employees are there?" (which would be 75) instead of the actual question — how many MORE factory than office. The answer they want is just k, which equals 15. Also, some students forget to apply the 5:4 ratio at all and treat F and O as independent variables, leading to an underdetermined system.
The elegant insight here: since F - O = 5k - 4k = k, you don't even need to separately compute F and O. As soon as you find k, you have the answer.
Takeaway: In ratio-word problems, assign variables as multiples of the ratio immediately (5k, 4k rather than two separate unknowns) — it often reduces the algebra significantly.