When will the reserves be exhausted? A rough estimate of the oil and gas reserves in some country at the beginning of 2010 was 15 billion tons. production that year was approximately 250 million tons.

when will the reserves be exhausted if production is kept at same level?

I have no clue on how to solve and tackle this problem, it seems kind of odd to me that there is nothing to indicate at which level the reserves are being consumed.

Really appreciate any hints on how to solve it.
If the country converts the reserves to 250 million per year, then in 4 years it will convert 1 billion and 15 billion will be exhausted in 15x4 years.