A town has found an oil field holding 1,000 million barrels. Two rigs start pumping. The oil sold pays for new rigs, and rigs wear out.
The emptier the field gets, the harder each rig has to work for the same oil. You decide how much of the money goes on new rigs.
Goal: The town needs at least 15 million barrels a year to live on. Keep it going for as many of the 50 years as you can.