A balloon is rising vertically upwards at a velocity of 10 ms^-1. When it is at a height of 45 m from the ground, a parachutist bails out from it. After 3s he opens his parachute and decelerates at a constant rate of 5 ms^-2. What was the height of the parachutist above the ground when he opened his parachute? (Take g = 10 ms^-2)

When the parachutist bails out, he shares the velocity of the balloon and has an upward velocity of 10 ms^-1, i.e. u= +10 ms^-1. 
Also g = -10 ms^-2 (acting downwards). The displacement in t = 3s is given by:

$$\begin{gathered} \mathrm{s}=\mathrm{ut} + \frac{1}{2}{\mathrm{gt}}^2 \\ =10\times 3+\frac{1}{2}\times \left(-10\right)\times {\left(3\right)}^2 \\ = -15 \mathrm{m} \end{gathered}$$

Since the displacement is negative, it is directed downwards. So the height from the ground when he opened his parachute = 45 - 15 = 30 m.