a football team started with the football at the 25 yard line,advancing toward the center of the filed ( 50…

a football team started with the football at the 25 yard line,advancing toward the center of the filed ( 50 yard line). Four successive plays resulted in a 5 yard gain, a 7 yard loss, a 6 yard gain and a 18 yard loss. what was the location of the football at the end of the fourth play?