The answer is 32 because you have to the inverse operation so you divide 320 by 2 you get 160. Then you divide 160 by 5a then you have a=32. To check you substitute: 5(32)2=320.
If a runner is behind you, s/he must have taken longer, so the times of all of the runners who finished after you did must be greater than 45.5 minutes
