please help me to understand this conculations? I can't figure it out what's going on there

Start with the equation, and remember your logs. 1,000 = 2011 - 1/1.015) / .015 ..75 = 1-1/ 1.0157 .1/ 1.0151 = .25 1/ .25 = 1.015 t = In(1/.25) / In(1.015) = 93.111 months = 7.76 years . And this is only if you don't charge anything more on the card! 6F-29