suppose you own a coal mine, you can sell the coal this year for price, p1, or next year, p2. the marginal cost of extraction is constant and the same each year, to max your profit over the two periods at the discount rate r you would prefer to:

extract and sell all the coal this year if p1-MC>(p2-MC)/(1+r)

extract and sell all the coal this year if p1-MC<(p2-MC)/(1+r)

extract and sell all the coal next year if p1-MC>(p2-MC)/(1+r)

leave the coal in the ground if p1-MC=(p2-MC)/(1+r)