Answer
$t=1.4~s$
Work Step by Step
We know that
$v_f=30mi/h$
$v_f=\frac{30\times 1.609\times 10^3}{3600}m/s$
$v_f=13.4m/s$
Now $v_f=v_{\circ}+gt$
This can be rearranged as:
$t=\frac{v_f-v_{\circ}}{g}$
We plug in the known values to obtain:
$t=\frac{13.4m/s-0m/s}{9.8m/s^2}=1.4s$
