Just looking at #2 and how to use the rule of 70 or compounding growth rate equation? Please help

D) labor productivity 2) If GDP is currently $15 trillion and is growing at a rate of 3.5% per year, how long will it take GDP to reach $30 trillion? A) about 10 years B) about 20 years C) about 7 years D) about 14 years inflation tion growth rates of 2%