Suppose the world's current oil reserves are 1820 billion barrels. If, on average, the total reserves is decreasing by 25 billion barrels of oil each year:
a. Give a linear equation for the remaining oil reserves, $R$, in terms of $t,$ the number of years since now.
b. Seven years from now, what will the oil reserves be?
c. If the rate of depletion isn't change, when will the world's oil reserves be depleted?