Let us assume that amount invested = 100
It quadruples meaning , it becomes = 100*4 = 400
We use the future value formula here
FV = PV*(1+r)^n
here FV = 400
PV = 100
n = 18 months or 18/3 = 6 quarters (Since we need a three month rate)
400 = 100*(1+r)^6
4 =1*(1+r)^6
r = 4^(1/6) -1
r = 1.2599 -1 = 0.2599
Interest rate being offered = 25.99% (Rounded to 2 decimals)