The rate percent on simple interest required to double a sum of money in 30 years is 3.33%.
A=P+SI------(1)
sum of money after 30 years = Double the money
A=2P----------(2)
Equate 1 and 2
P+SI=2P
SI=2P-P
SI=P-------------(3)
WKT, SI= PTR/100
Equate 3 and SI
P=PTR/100
P=P*30*R/100
R=P*100/P*30
R=100/30 or 10/3 or 3(1/3) %