Correct Answer: 2 mph
Explanation:
Speed downstream = (10 + x) mph,
Speed upstream = (10 - x) mph.
⟹ 72x x 60 = 90 (100 - x2)
⟹ x2 + 48x - 100 = 0
⟹ (x+ 50)(x - 2) = 0
⟹ x = 2 mph.