Explanation:
Two periods (semi-annual) per year
decimal interest per period = i = .03/2 = .015
pv = present value = 750
fv = future value = 1500
fv = pv (1 + i ) ^2t where t = years
1500 = 750 ( 1 + .015)^2t <==== solve for t
2 = 1.015 ^(2t)
log 2 = 2t log (1.015)
t = 23 years