We have the following information; 
The world's current oil reserves is R= 1830 billion barrels and is decreasing by 20 billion barrels of oil a year. 
a. We want to give an equation for the remaining barrels R in terms of time t. 
The equation is ;

b. 8 years from now, the world's oil reserve would be; 

c. If no other sources are discovered, the world's oil will be finished when R is zero. 
Thus;
