Bouncing-ball simulation
Comments

You should use // comments for single-line comments.

I am not sure you are using comments properly. Comments shouldn't explain how, it should explain why.

/* Prototype for drop_Balls, int parameter is number of balls being dropped */
void drop_balls(int); 


One can see that this is a prototype. void drop_balls(int number_of_balls); would probably provide as much information in a more compact way.

/* Return the number of balls that will be dropped */
return num_balls; 


This does not look like a useful comment to me.

/* Keeps track of how many balls landed where */
int ball_count[21]; 


A bit of explanation on the 21 here would be useful... but it is not provided.

This leads me to the next section:

Magic numbers

You probably don't want to have hard-coded numbers all over the place. You could create a constant with value 21 and then:


starting_point = 10 would become starting_point = N /2.
for(x = 0; x &lt; 20; x+2) would become for(x = 0; x &lt; N; x+2).
int ball_place = -10; would become int ball_place = - N/2;


Code organisation

It would probably be better now to call srand in rand_balls as it shouldn't be its responsibility. You can do it from main() if you want.

Define variable in the smallest possible scope. For instance:

for(int ball_num = 0; ball_num &lt; num_balls; ball_num++ )


Also, how is this loop:

for(starting_point = 10; starting_point &gt; 0; starting_point--)


supposed to behave?  Don't we always have the variable set to 0 or -1 at the end?