T=2 *pi*sqrt{l/g}
N=t/T
N2/N1=t*T1/(t*T2)=T1/T2=2 *pi*sqrt{l1/g}/(2 *pi*sqrt{l2/g})=sqrt{l1/l2}=sqrt{l1/(l1+0,15)}
5/10=sqrt{l1/(l1+0,15)}
2^2=(l1+0,15)/l1
4=1+0,15/l1
0,15/l1=3
l1=0,15/3
l1=0,05м
l1=5cм
l2=5+15=20cм
T=2 *pi*sqrt{l/g}
N=t/T
N2/N1=t*T1/(t*T2)=T1/T2=2 *pi*sqrt{l1/g}/(2 *pi*sqrt{l2/g})=sqrt{l1/l2}=sqrt{l1/(l1+0,15)}
5/10=sqrt{l1/(l1+0,15)}
2^2=(l1+0,15)/l1
4=1+0,15/l1
0,15/l1=3
l1=0,15/3
l1=0,05м
l1=5cм
l2=5+15=20cм