$150,000 * (1 + X)^8 = $300,000
(1 + X)^8 = $300,000/$150,000
(1 + X)^8 = 2
{(1 + X)^8 }^0.125 = 2^0.125
1 + X = 1.0905
X= 9.05% 利率有點高了些