Question

in how much time will a certain sum be 1.5 times the principal at 10%per annum?

Answer 1

Let the principal be 100
Amount = 1½100= 300/2 = 150
Interest = 150-100 = 50
But, interest = PRT/100 = 100 × 10 × T /100
=>10T
A/Q, 10T = 50
=> T = 5
Required time = 5 years

Answer 2

Assuming that you are asking for Simple interest:
Let the principle amount be Rs.100
Therefore,Amount after certain years will be 1.5*100=Rs.150 
Amount= P + SI=P+(PRT/100)
150=100+(100*10*T/100)
150-100=10T
10T=50
T=5
Therefore,time is 5 years.