Start with 15 m/s. There are 2 units and you need to cancel the seconds to get meters.
10 seconds has seconds as a unit. This can be used to cancel the sec in the denominator of 15 m/s. This just a little tip I use when I'm unsure about what to do.
Answer: The ice cube would float on top of the water and the rock would sink to the bottom.
Explanation: The ice cube has a smaller density than the rock which allows the ice cube to float but makes the rock sink to the bottom of the glass of water.
