Math+(making+water+last)

Steps: 1. 128 500ML Bottles of Water 2. 128/.75 3. 32 4. 32 X 500 5. 16000 ML 6. 16000/100 7. 16 Liters of Water 8. 16/4 9. 4 Liters per person 10. Water for 11.5 days per person

4 people can last 11.5 days with water in 90 degree weather.