Question: 2014 Fall Final - 7
Consider $m \geq 7$ blue balls $B_1,B_2,\dots,B_m$ and $n \geq 7$ red balls $R_1,R_2,\dots,R_n$.
We pick 7 balls of the same color and arrange them on a horizontal line. (The order on the line
matters.) How many arrangements are there?
(a)
$7!{m + n \choose 7}$
(b)
None of the above.
(c)
$m!{m \choose 7} + n!{n \choose 7}$
(d)
$7!{m \choose 7} + 7!{n \choose 7}$