Interest
1 percent compound interest on $100 will cause the amount to double to $200 in about 72 years. That is called the rule of 72 which can be used to figure out how fast something grows. The rule can be used to figure out how fast anything that always  grows exponentially at the same rate will double in size. Suppose a kangaroo grows 12 percent a year; dividing 12 into 72 gives 6, so the kangaroo will double in size in 6 years. If the kangaroo grew 6 percent a year it would take 12 years for its size to double. If it only grew 2 percent it would take 36 years for its size to double.