The sum gets doubled in 5 yr and triplled in 12 yr.
So from the given data, its clear that rate of interest for 12 yr is lower.
Let principal be x.
WKT, SI = PNR/100, SI = 2x
R = (SI*100)/PN
= (2x * 100)/(x * 12)
= 50/3 = 16(2/3)%
Therefore, lower rate of interest per annum is 16(2/3)%.