average growth rate = 126539 / 24 = 5272.45833333... bacteria / hour
Let x(0) = 400 = number of bacteria at time t = 0. It is bad to say r(0) = 400, because r(t) is already defined as rate.
We can use average growth rate to find the number of bacteria by taking x(24) = x(0) + (24 * average).
Or we can simply use the intermediate result from before and say that x(24) = x(0) + 126539 = 400 + 126539 = 126939 bacteria after 24 hours.
As I said in another post to a similar question of yours just now, I'm a bit rusty, so I wouldn't mind if someone else confirmed my results.