The basic equation that you need to work is this:
where N = the number of 5-minute periods. You can manipulate it as follows:
Now you have N, and the time required is 5 minutes times that.
"Increases by 25%" means it goes from A to A+ .25A= (1.25)A every 5 minutes. That is, every 5 minutes you multiply by 1.25:
If it starts at 400, in 5 minutes it will be 400(1.25). After 10= 2(5) minutes it will be multiplied by 1.25 again- it will now be . After 15= 3(5) minutes it will be multiplied by 1.25 another time- it will now be . In t minutes there will be N= t/5 periods of 5 minutes and so the 400 will have been multiplied by 1.25 t/5 times giving ebaines'