You can form either 1,000 or 750 3 digit passwords using the numbers 0 through 9. If you use numbers that can be repeated at least once, you can get up to 1,000 unique passwords. If you only use numbers that do not repeat at all, you get up to 750 unique passwords.
Problems of this sort are frequently found in physics. If you study calculus or physics you'll learn how to create the equation representing the velocity of an object in flight.
Here, you don't need to calculate velocity, but rather time. Start with this equation:
v = v0 + a t^2, where v is the velocity at time t, v0 is the initial velocity, a is the acceleration due to gravity (denoted by g instead of a), and t is the elapsed time.
You are told that v0 is 15 ft/sec. Set v = to 0, as the ball stops moving for the tiniest instant at the top of its trajectory. Use g = - 32 ft (per second squared).
Then 0 = 15 ft/sec - 32 [ft/(seconds squared)] t.
Solve this for t. This is the time required for the ball to come to a complete stop at the top of its trajectory.
Finally, multiply this time by 2, since the ball begins to fall and returns to its original height.
Answer:
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