Answer and Step-by-step explanation: Paddling 12 miles upstream will take you 12 mi / (vp - 2 mi/hr) = 12 / (vp-2) hr. (If you're having trouble figuring out how to calculate time from speed and distance, just look at the units. Time (hr) is required as a result. We have two units: miles and miles per hour. Divided by mi/hr, mi is equal to mi * hr/mi, which equals hr. 28 / (vp + 2) is the time it takes to paddle 28 miles downstream. Also, these ought to be equal: 12 / (vp-2) = 28 / (vp+2) (cross multiplication) (cross-multiplication) 28 * (vp-2) = 12 * (vp+2) 28 vp - 56 = 12 vp + 24 (collect vp terms on one the left side and constants on the right side) (collect vp terms on one the left side and constants on the right side) (28-12) vp = 24 + 56\svp = 80/16