Jack is practicing prior to a football tryout. He wants to work on his speed and agility, so he practices running forward the length of a football field and then running backwards the length of a football field. When he runs forward, he can run 100 yards in 10 seconds. When he is running backward, he can run 100 yards in 20 seconds. What is his speed when he is running backward?
1) 10yd/sec
2) -5yd/sec
3) -10 yd/sec
4) 5 yd/sec