Compound interest formula: How long does it take to double a $1,000 investment that pays 6.5% annual interest, compounded monthly? Which equation can you use to solve this problem? 1000 = (1 + StartFraction 0.065 Over 12 EndFraction) Superscript t 2 = (1 + StartFraction 0.065 Over 12 EndFraction) Superscript 12 t 2 = (1 + StartFraction 6.5 Over 12 EndFraction) Superscript 12 t