David and Sarah are both saving money to buy a new bicycle. David starts with $100 and saves $15 each week. Sarah begins with $50 and saves $25 each week.

To find out when they will have the same amount of money saved, we can set up a system of linear equations. Let $d$ represent the amount of money David has saved after $x$ weeks, and $s$ represent the amount of money Sarah has saved after $x$ weeks.

Which value of $x$ will make $d = s$?