From the 15 yard line to the 50 yard line (the center) is 35 yards. It's another 10 yards to the other team's 40 yard line. So the total distance from one team's 15 yard line to the other team's 40 yard line is 45 yards.
The kickoff is from the kicking team's 30 yard line making the kickoff travel 70 yards to reach the end zone.
The 20 yard line
The ball is teed-up at the 40-yard line of the kicking team.
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40 yards
The 30-yard line of the team kicking the ball off.
The "red zone" is generally defined as the area between the opponent's 20 yard-line and the goal line. Note that there is no rule explicitly defining this; it's just an expression made up by announcers and fans.
The player has run 40 yards.
A loss of yardage in football is when someone on the team holds the football in his/her hand(s)/arm(s) and someone on the other team tackles the person with the football before the person with the football can advance across the line in which the play started. For example: the team with the football start on the fifty yard line and they need to advance the ball to the forty yard line for a first down. The football gets snapped to the quarterback. The quarterback has two options: run with the football or throw the football to someone on his/her team so the team can attempt to get the ball across the forty yard line. The quarterback notices that someone on the other team is running at him/her and runs backwards to the forty yard line. Before the quarterback has the chance to either run with the football or throw the football, the person tackles the quarterback at the forty yard line resulting in a ten yard loss.
It depends on the league and level of football, but in the NFL, the football is normally placed on the 2 yard line for both a point-after kick or a 2-point conversion attempt. In high school, it's placed on the 3 yard line. CFL rules put it on the 5 yard line.
40 yards