Answer:
20, 000
Step-by-step explanation:
For a duration of 50 years, the consumption will be equal to the product of annual consumption and the number of years hence 800*50=40, 000 barrels.
Since 20, 000 barrels are unknown and they are part if consumption in the next 50 years, then the known or proven reserves will be given by getting the difference between total consumption and unknown reserve, expressed as 40, 000-20, 000=20, 000 barrels.