You and your neighbors are considering the installation of automatic hand sanitizer dispensers in the building due to the pandemic. Each resident is willing to pay $9 for each additional automatic hand sanitizer dispenser installed in the building. There are 2 residents living on each floor and there are 10 floors in the building. The marginal cost of installing another automatic hand sanitizer dispenser is given by 100 + 2Q. The efficient number of automatic hand sanitizer dispensers to be installed in the building is [Answer].