A gas balloon is going up with a constant velocity of 10 m / s. When this balloon reached a height of 75 m, a stone is dropped from it and balloon…

A gas balloon is going up with a constant velocity of $\displaystyle 10 \mathrm{~m} / \mathrm{s}$. When this balloon reached a height of $\displaystyle 75$ m, a stone is dropped from it and balloon keeps moving up with the same velocity. The height of the balloon when the stone hits the ground is $\displaystyle \_\_\_\_$ m. (Take $\displaystyle g=10 \mathrm{~m} / \mathrm{s}^2$ )