The annual flow of rivers in the world is 43,780 km3 of water. 425 km3 of waste water is discharged into rivers every year.

The annual flow of rivers in the world is 43,780 km3 of water. 425 km3 of waste water is discharged into rivers every year. 1 m3 of waste water pollutes on average 50 m3 of clean water. What proportion of the world’s river flow is polluted?

1) 1 m3 pollutes 50 m3, therefore, 1 km3 will pollute 50 km3 of water.
2) Find the amount of polluted water per year:
425 km3 × 50 km3 = 21 250 km3
3) Find the ratio between river runoff and polluted waters (in%):
21,250 km3: 43,780 km3 × 100% ≈ 48.5%
Answer: ≈ 48.5% of the world river runoff is polluted.

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