I think I have this solved . . .
Assuming she throws the stone horizontally, your calculations are correct. A girl stands at a waters edge and throws a stone from a height of 0.9 m.
To get the stone to bounce, it must hit the water at an angle of 15°.
Find the minimum speed which she can throw the stone.
Well, i calculated the time for the stone to fall: 0.429 seconds (correct in the answers).
The vertical component of the stone hitting the water, the speed is 4.2 m/s.
As you found, the vertical component of the speed is: m/sec.
Let the original (horizontal) speed be . .This is constant throughout the flight.
When the stone strikes the water, we have this diagram:
* - - - - - - - - - - - *
* 15° |
* | 4.2
In the right triangle: .
Therefore: . m/sec.