A municipal water tank contained 2,500,000 liters of water at midnight. At 6:30 a.m., the tank contained 2,383,000 liters. On...

1. TRANSLATE the problem information

• Given information:
  • Initial amount: 2,500,000 liters (at midnight)
  • Final amount: 2,383,000 liters (at 6:30 a.m.)
  • Time period: midnight to 6:30 a.m.
  • Need to find: average liters used per minute

2. INFER the solution strategy

• This is a rate problem: \(\mathrm{Rate = Quantity ÷ Time}\)
• We need to find how much water was used and how much time elapsed
• Time must be in minutes since we want liters per minute

3. SIMPLIFY the time calculation

• Time elapsed: midnight to 6:30 a.m. = 6 hours and 30 minutes = 6.5 hours
• Convert to minutes: \(\mathrm{6.5 × 60 = 390}\) minutes

4. SIMPLIFY to find water used

• Water used = Starting amount - Ending amount
• Water used = \(\mathrm{2,500,000 - 2,383,000 = 117,000}\) liters

5. SIMPLIFY to find the rate

• Rate = Water used ÷ Time elapsed
• Rate = \(\mathrm{117,000 ÷ 390 = 300}\) liters per minute (use calculator)

Answer: A (300)

Why Students Usually Falter on This Problem

Most Common Error Path:

Weak INFER skill: Students recognize this is a rate problem but fail to convert hours to minutes, working directly with 6.5 hours instead of 390 minutes.

They calculate: \(\mathrm{117,000 ÷ 6.5 = 18,000}\) liters per hour, then mistakenly think this is the per-minute rate.

This leads them to select Choice D (18,000).

Second Most Common Error:

Poor SIMPLIFY execution: Students set up the problem correctly but make arithmetic errors in the final division, either through calculator mistakes or approximation errors.

This causes them to get confused about which answer choice is correct and may lead to guessing among the remaining choices.

The Bottom Line:

The key challenge is remembering that rate problems require matching time units - if you want "per minute," your time must be in minutes, not hours. Always check your units!