Answer
Approximately $329.4ft$
Work Step by Step
Let the maximum range of projectile be $R$
Velocity be $v$
And the constant of proportionality $k$
With the given information, we can write the relation:
$R=kv^2$
$v=60mi/h$
$R=242ft$
$k$ is unknown
$242=k\times 60^2$
$k=\frac{242}{3600}=\frac{121}{1800}$
If $v=70mi/h$
$R=\frac{121}{1800}\times4900=\frac{5929}{18}\approx 329.4ft$
