Answer
$\approx7.4$
Work Step by Step
$v_{i} = 0 , v_{f} = v , v_{rel} = u $
$v_{i} $ is the initial speed.
$v_{f} $ is the final speed.
$v_{rel} $ is the relative speed and
u is the exhaust speed.
$v_{f} - v_{i} = v_{rel} ln\frac{M_{i}}{M_{f}} $
$=> \frac{M_{i}}{M_{f}} =e^{\frac{v}{u}}$
(b) If v = 2u,
$\frac{M_{i}}{M_{f}} =e^{2}\approx 7.4 $
