A boat goes downstream and covers 84 km distance in 12 hours and when it goes upstream, cover half of the downstream distance at the same time. Find the speed of the current.

Downstream: Distance = 84 km, Time = 12 hours, so downstream speed = 84/12 = 7 km/h = (b + c) where b is boat speed and c is current speed. Upstream: Distance = 42 km (half of 84), Time = 12 hours, so upstream speed = 42/12 = 3.5 km/h = (b - c). Subtracting: (b + c) - (b - c) = 7 - 3.5, so 2c = 3.5, giving c = 1.75 km/h = 7/4 km/h.