After 5 minutes the pool will have 249,000 gallons since 5x200=1000 and 250,000-1000=249,000. After 20 minutes it will be at 246,000 gallons since 20x200=4000 and 250,000-4000= 246,000. Hope this helps :)
To get the amount you need to subtract from 840, just turn 5% into a decimal, which is 0.05, then just multiply 0.05 by 840 and subtract that number from 840