Each resident is willing to pay $10 for each additional streetlight installed in the neighborhood. There are 15 residents in the neighbourhood. The construction cost of installing streetlights is x², where x is the number of streetlights. So the private marginal cost of installing another streetlight is given by 2x. In addition, each streetlight is estimated to bring $50 environmental cost to the nieghborhood. The efficient number of streetlights to be installed in this neighborhood is: