This is like the equivalent to a jar with 4 green balls and 6 white balls, where you are picking 3. (The 4 green balls signify the friends from kindergarten.)
You want to solve the probability that the first two balls are green and the third is white.
First draw --> 4 green out of 10 balls --> 4/10 = 2/5
Second draw --> 3 green out of 9 balls --> 3/9 = 1/3
Third draw --> 6 white out of 8 balls --> 6/8 = 3/4
2/5 x 1/3 x 3/4
= 6/60
= 1/10
so the answer is 1/10 (or 10%)
140-525 would be the range
The speed of the ball is
ds/dt = 32t
At t =1/2 s
ds/dt = 16 ft/s
The distance from the ground
50 - 16(1/2)^2 = 46 ft
The triangles formed are similar
50/46 = (30 + x)/x
x = 345 ft
50 / (50 - s) = (30 + x)/x
Taking the derivative and substituing
ds/dt = 16
and
Solve for dx/dt
