Answer
(a) Two
(b) $3.8~s$
(c) $7.6~s$
Work Step by Step
(a) $a=\frac{\Delta v}{\Delta t}$. Rearrange the equation: $\Delta t=\frac{\Delta v}{a}$.
So, time is directly proportional to the change of velocity. That is, if $\Delta v_{2}=2\Delta v_{1}$, then $\Delta t_{2}=2\Delta t_{1}$.
(b) $\Delta t_{1}=\frac{\Delta v_{1}}{a}=\frac{v_{f}-v_{i}}{a}=\frac{0-16~m/s}{-4.2~m/s^{2}}=3.8~s$
(c) $\Delta t_{2}=\frac{\Delta v_{2}}{a}=\frac{v_{f}-v_{i}}{a}=\frac{0-32~m/s}{-4.2~m/s^{2}}=7.6~s$
