Sorry for the confusion, yeah I forgot how to do basic algebra and don't understand why this is the answer: l=4π^2gT^2, and not this l=gT^2/4π^2 (what you said and what I think it is too)?
Either way you can still find the answer because when you multiply T by 0.80, you multiply l by 0.64...
Homework Statement
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?
Homework Equations
T=2π√(l/g)
T=period
l=length of the rod
g=acceleration due to gravity
The Attempt at a Solution
I know...