What is the Rule of 72, and how do you calculate it? a An estimate for the number of years it will take for the principal deposit to double, calculated by dividing 72 by the rate of return. b An estimate for the number of years it will take for the rate of return to double, calculated by dividing 72 by the length of time the principal amount is held on deposit. c An estimate for the number of years it will take for an investment to double, calculated by subtracting the rate of return (interest rate) from 72. d An estimate of the time it will take you to graduate from a 4-year college, you divide 72 by the total number of courses you need to graduate.

Asmall town wants to build some new recreational facilities. the proposed facilities include a swimming pool, recreation center, basketball court and baseball field. the town council wants to provide the facilities which will be used by the most people, but they face budget and land limitations. the town has $400,000 and 14 acres of land. the pool requires locker facilities which would be in the recreation center, so if the swimming pool is built the recreation center must also be built. also, the council has only enough flat land to build the basketball court or the baseball field. the daily usage and cost of the facilities (in $1,000) are shown below. which facilities should they build? facilityusagecost ($1,000)landswimming pool4001002recreation center5002003basketball court3001504baseball field2001005