Essential University Physics: Volume 1 (4th Edition)

We first must find the time of the new period: $T = 6 \times \sqrt{\frac{90+20}{60+20}}=6 \times \sqrt{\frac{110}{80}}=7.04 \ s$ Thus, we use the equation for percent increase to obtain: $=\frac{7.04-6}{6}=.1726 = \fbox{17.26 percent}$ Thus, c is the correct answer.