Mr. Dong has hired his neighbor, Robin, to look after his salt water fish tank while he travels the world. The tank holds 10 gallons. When first set up, Mr. Dong put 11 pounds of salt in the 10 gallon tank to create the correct salinity. It is good practice in keeping fish to replace 10% of the water each week. Also each week, one gallon of water evaporates, but no salt is lost, so the tank would then contain 11 pounds of salt in only 9 gallons.

After one week, Robin proceeds to add enough fresh water to replace the evaporated water, then removes 1 gallons of tank water and adds 1 gallons of fresh water. Robin is now creating a problem for the fish in that the salinity of the water has been reduced.

Robin's friend, Naomi, realizes that removing the water also removes the salt. She suggests he add a bit of salt with each water change. They decide on this plan: remove 1 gallons of water and add 0.3 pounds of salt. Unfortunately, they did not take into account evaporation, so they were still changing the salinity of the tank.

Determine the amount of salt remaining in the tank at the end of each exchange with this new plan.

After 0 weeks: 11 pounds
After 1 week:
pounds
After 2 weeks:
pounds
After 3 weeks:
pounds
After 4 weeks:
pounds
After 5 weeks:
pounds
After 10 weeks:
pounds
After 15 weeks:
pounds
After 20 weeks:
pounds

After how many weeks are there less than 4 pounds of salt in the tank under the new system?