#### Answer

His car can go 150 miles per hour.

#### Work Step by Step

We need to convert 0.25 miles per 6 seconds into miles per hour.
To do this, we multiply 0.25 miles per 6 seconds by the ratio that converts seconds to hours.
The ratio that converts seconds to hours is $\frac{3600\ seconds}{1\ hour}$.
So, $\frac{0.25\ mile}{6\ seconds}$ $\times$ $\frac{3600\ seconds}{1\ hour}$ =
$\frac{900\ miles}{6\ hours}$ =
150 miles per hour.