Reading a School Waste Audit Report
A school runs a waste audit. The report states: "In Week 1, the canteen produced 18 kg of food waste and the classrooms produced 12 kg. In Week 2, after a 'scrape your plate' campaign, canteen waste fell to 11 kg while classroom waste rose to 15 kg. The campaign poster said, 'Our campaign cut school waste by a third!' A separate note records that Week 2 had a public holiday on Friday, so only four school days were recorded, compared with five in Week 1." A student reads the poster and says the school now wastes less overall.
1. Calculate total waste for each week and decide whether the poster's claim of a one-third cut is accurate.
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Week 1 total: 18 + 12 = 30 kg. Week 2 total: 11 + 15 = 26 kg.
Reduction = 30 − 26 = 4 kg. As a fraction of Week 1: 4 ÷ 30 ≈ 0.133, or about 13%, not one third (33%).
The poster's claim is inaccurate on the raw totals. A one-third cut would require a fall of 10 kg, to 20 kg. The campaign did reduce total waste, but by roughly 13%, and the canteen fall was partly offset by a rise in classroom waste.
2. Explain how the four-day week affects the comparison, and recalculate using a daily average.
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Comparing a 5-day week with a 4-day week is unfair because fewer days naturally produce less waste. Use a daily average:
Week 1: 30 kg ÷ 5 days = 6.0 kg per day. Week 2: 26 kg ÷ 4 days = 6.5 kg per day.
On a per-day basis, waste actually rose from 6.0 kg to 6.5 kg, an increase of 0.5 kg per day (about 8%). So the campaign did not reduce waste per school day; the apparent drop is explained by the missing Friday. This is the key judgement: totals can mislead when the time periods differ.
3. The canteen waste fell but classroom waste rose. What does this suggest about the campaign, and what evidence would you want next?
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The campaign targeted plate scraping, which directly addresses canteen food waste, so the canteen fall (18 kg to 11 kg) is consistent with the campaign working where it was aimed. Classroom waste rose (12 kg to 15 kg), which suggests the campaign did not reach classrooms, or that another factor (such as a craft activity or new paper use) increased classroom waste.
A reasonable inference is that the campaign had a local effect, not a school-wide one. To confirm, I would want: daily waste weights for each area across several weeks, a count of students present each day, and a record of any classroom activities that week. Without a comparison week of the same length, the per-day figures remain the best evidence.