First find the interest rate of the sum of money doubles in 20 years
2p=p (1+rt)
2=1+rt. T=20 years
2=1+20r
Solve for r to get the simple interest rate
2-1=20r
20r=1
R=1÷20
R=0.05
Now find how long will get triple at the same rate
3p=p (1+rt)
3=1+rt. R=0.05
3=1+0.05t
Solve for t
3-1=0.05t
0.05t=2
T=2÷0.05
T=40 years. .....answer
