The marginal social benefit of timber is p = 70 - 1Q, where Q is measured in thousands of tonnes of timber per day, and price is measured in dollars per tonne per day. The marginal private cost of timber extraction is $12 per tonne per day. The marginal external cost of timber extraction (measured in dollars per tonne per day) is MEC = 1Q. The socially optimal price is __________ dollars per tonne per day