A car travels 400 miles in 12 hours. What is its average speed?

The average speed is 33.333333 mph.

v = \frac{d}{t} = \frac{400}{12} = 33.333333\ \text{mph}

How to work it out

1
Write down what you know: distance d = 400 miles, time t = 12 hours. You are asked for the speed v.
2
Use the formula speed = distance ÷ time:
$v = \frac{d}{t} = \frac{400}{12} = \frac{100}{3} = 33.333333$
3
Answer with the unit: 33.333333 mph.
4
The same formula rearranged: distance = speed × time (33.333333 × 12 = 400 miles) and time = distance ÷ speed (400 ÷ 33.333333 = 12 hours).

LaTeX source of the formulas on this page

\[ v = \frac{d}{t} = \frac{400}{12} = 33.333333\ \text{mph} \]
\[ v = \frac{d}{t} = \frac{400}{12} = \frac{100}{3} = 33.333333 \]

What is the speed if a car travels 400 miles in 12 hours?

The average speed is 33.333333 mph.

How far does it go in 3 hours at that speed?

100 miles.