Let the one-way distance be D km.<br><br>&bull; Downstream speed = 12 + 2 = 14 km/h, time = D/14.<br>&bull; Upstream speed = 12 − 2 = 10 km/h, time = D/10.<br>&bull; Total time = 6 h.<br><br>D/14 + D/10 = 6<br>D × (1/14 + 1/10) = 6<br>D × ((10 + 14)/140) = 6<br>D × (24/140) = 6<br>D = 6 × 140/24 = 6 × (35/6) = 35 km.<br><br>Final Answer: 35 km