Hot air balloon a is rising upward at a rate of 10.0 m/s. a passenger aboard balloon a observes balloon b going down at a relative rate of 12.5 m/s. what is the velocity of balloon b relative to the ground? assume that up is positive.

Since balloon A is going up at 10 m/s and balloon B looks like it is going down 12.5 m/s , then from the ground it will look like the balloon B is coming down at 12.5 - 10 = 2.5 m/s

Think of it like this. If there was a stationery point on the cliff next to a balloon, the point on the cliff would appear to be going down at 10 m/s to balloon A even though it is stationary. So if it looks like it s 12.5 m/s to the balloon going up then to someone on the ground it is only dropping another 2.5 m/s

The answer should be a.