Let the function f be defined by $f(x)=10x-2a$, where a is $a$ constant . If $f(5)+f(10)=55$ What is the value of $a$?

Question

$\begin{array}{1 1} (a)\;23 \\ (b)\;23.25\\ (c)\;23.50\\(d)\;23.75 \end{array}$