Let's call Maria's average speed

Imagine for a moment that Maria has a stopwatch, that measures in hours.
She starts her stopwatch when she starts driving.
If Maria looks at her stopwatch at a time

, her distance traveled will be

miles. (This is true since

.)
What about Ralph? If Ralph could look at Maria's stopwatch, his distance traveled would be

, since his speed is 40mph and he got a six-mile head start. Remember that

is measured from Maria's start, not Ralph's.
So when they meet, after 1.5 hours, their distances are equal. So:


And since that time is 1.5, we plug that in for t:



Divide both sides by 1.5:

Which cancels on the left:

miles per hour.