1. The problem statement, all variables and given/known data The length of the rod of a certain pendulum is decreased, and the period then decreases by 20%. By how much was the rod length decreased? 2. Relevant equations T=2π√(l/g) T=period l=length of the rod g=acceleration due to gravity 3. The attempt at a solution I know how to solve the problem, I just don't know how to do the math to solve for l (length).