Let speed of stream = a km/h
∴ Downstream speed = (15+a) kilometre/hour
∴ Upstream speed = (15-a) kilometre/hour
∴ 30/(15+a) + 30/(15-a) = 4 1/2
→ 900/225-x² = 9/2
→ 9x² = 225
→ x² = 25
→ x = 5
∴ Speed of stream = 5 km/h
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