Answer
Samantha has $8$ roses.
Work Step by Step
Let $m$ denote the number of roses Mary has, $j$ denote the number of roses Jack has, $s$ denote the number of roses Samantha has. We know that $m=5,j=7$ and that $m+j+s=20$.
Thus: $m+j+s=20\\5+7+s=20\\12+s=20\\12+s-12=20-12\\s=8$
Thus Samantha has $8$ roses.
