The chance of student 1's birthday being individual is 365/365 or 100%. Then the chance of student 2's birthday being different is 364/365. Then it's narrowed down to 363/365 for student 3 and so on until you get all 10 students. If you multiply all these values together, the probability would come out at around 0.88305182223 or 0.88.
To get all the same birthday you'd have to the chance of one birthday, 1/365 and multiply this by itself 10 times. This will produce a very tiny number. In standard form this would be 2.3827x10'-26 or in normal terms: 0.23827109210000000000000000, so very small.
When stone hits the ground, it's height will be zero, and since we're finding the time that's required for the stone to hit the ground, we can set h = 0 and solve for t.
