Hot air balloon A is rising upward a rate of 5.0 m/s. A passenger aboard balloon A observes balloon B rising upward at a relative rate of 7.5 m/s. What is the velocity of balloon B relative to the ground? Assume that up is positive.

Velocity B (relative to the A) = Velocity B (relative to the ground) - Velocity A (relative to the ground)

Velocity B (relative to the ground) = Velocity B (relative to the A) + Velocity A (relative to the ground)

Velocity B (relative to the ground) = 7.5 + 5 = 12 m/s

Since both go up in the same direction, we should use the minus sign for relative velocity.