Determining Cost of Standard Membership

A bookstore sells two types of memberships: Standard and Premium. The cost of a Standard membership is $$ S $$ dollars, while a Premium membership costs $$ P $$ dollars. The bookstore's linear pricing strategy can be modeled by the equation:

$$ C(x) = 5x + 15 $$

where $$ C $$ represents the total cost in dollars for purchasing $$ x $$ memberships. If a customer buys 4 memberships, they want to know the total cost associated with this linear function. Additionally, the cost of one Premium membership is twice that of one Standard membership. Determine the cost of a Standard membership.