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'Pied Piper' printed from https://nrich.maths.org/
Answer: 3736
Tuesday: 900 rats
Wednesday: 20% more than 900 = 900 + 90 + 90 = 1080 rats
Thursday: 30% fewer than 1080 = 1080 $-$ 108$\times$3 = 756 rats
Total: 1000 + 900 + 1080 + 756 = 3736 rats