Yes, besides reading the problem, doing the problem did not take long. If your amount doubles in 14 years, then 3k will reach 6k in 14 years. Then, it will take just another 15 years for 6k to double into 12k. 14+14=28
It may also be useful, although certainly outside of the GMAT scope, to know the rule of 72. If it takes 14 years to double an investment, then 72/14 = approx 5 which represents the interest rate.