Tom, Dorothy, and Sammy went on a vacation and agreed to split the costs evenly.  During their trip Tom paid $105, Dorothy paid $125, and Sammy paid $175.  In order to share costs equally, Tom gave Sammy $t$ dollars, and Dorothy gave Sammy $d$ dollars.  What is $t-d$?

$\textbf{(A)}\ 15\qquad\textbf{(B)}\ 20\qquad\textbf{(C)}\ 25\qquad\textbf{(D)}\ 30\qquad\textbf{(E)}\ 35$
The total amount paid is $105 + 125 + 175 = 405$.  To get how much each should have paid, we do $405/3 = 135$.
Thus, we know that Tom needs to give Sammy 30 dollars, and Dorothy 10 dollars.  This means that $t-d = 30 - 10 = \boxed{\textbf{(B) }20}$.