First collision

There are two objects $$A$$ ($$15 \text{ m}$$ above ground) and $$B$$ ($$10 \text{ m}$$ above ground) on a straight line perpendicular to the ground. $$A$$ is thrown straight downward with a speed of $$10 \text{ m/s}$$ and, at the same time, $$B$$ starts to fall freely. Find the time that passes until their first collision.

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