Aboat travels 30 miles upstream in 5 hours. the return trip downstream takes 3 hours. if the boat travels at the same rate on both parts of the trip, what is the rate of the current, in miles per hour?

Velocity against current = 30 miles / 5 hours = 6 mph
velocity with current = 30 miles / 3 hours = 10 mph

"bv" means boat velocity (no current)
bv (against) -current = 6 mph
bv (with) + current = 10 mph
Adding both equations:
2 * bv = 16 mph
boat velocity = 8 mph
Therefore current = 2 mph

Source:
http://www.1728.org/veloccal.htm

If gravity is one third on mars then weight also one third on mars

The following relation is not a function