1 .
Consider a group consisting of 7 girls and 6 boys. Elisa is one of the girls. How many ways are
there to arrange these 13 people on a horizontal line such that Elisa has 2 neighbors, both of whom
are girls? (The order on the line matters.)
(a)
$12 \cdot 6 \cdot 5 \cdot 10!$
(b)
$11 \cdot 6 \cdot 5 \cdot 10!$
(c)
$11 \cdot {6 \choose 2} \cdot 10!$
(d)
$13 \cdot 6 \cdot 5 \cdot 10!$