Question: how to calculate percentage of interest?
Answer: Calculate Total Amount Accrued (Principal + Interest), solve for A. A = P(1 + rt)
Calculate Principal Amount, solve for P. P = A / (1 + rt)
Calculate rate of interest in decimal, solve for r. r = (1/t)(A/P - 1)
Calculate rate of interest in percent. ... 
Calculate time, solve for t.