Answer
c)
Work Step by Step
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.
