Answer:
 A)  R = 1900 -24t
 B)  1684 billion barrels
 C) 79.17 years
Step-by-step explanation:
Reserves start at 1900 billion barrels and decrease by 24 billion barrels each year. The value for t=0 is 1900; the value for t=1 is 24 less.
 R = 1900 -24t
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For t=9, the reserves will be ...
 R = 1900 -24(9) = 1900 -216 = 1684 . . . . billion barrels
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The reserve will be 0 when ...
 0 = 1900 -24t
 24t = 1900 . . . . . add 24t
 t ≈ 79.17 . . . . divide by 24
Reserves will be gone in about 79.17 years.