Question

When mr. johnson drives from cambridge to newton it takes him 15 minutes travelling at a speed of 80 miles per hour. mrs. johnson drives from cambridge to newton and leaves 10 minutes after mr. johnson but travels at a speed of 100 miles per hour. who will arrive first? by how many minutes?

Answer 1

Given 
  15 minutes : 80 miles (Mr. Johnson)
  10 minutes : 100 miles (Mrs. Johnson)
Find: who will arrive first and by how many minutes
Solution
 15 - 10 = 5 minutes
 
The answer is that Mrs. Johnson will arrive the her destination first by less than 5 minutes from the traveling time of Mr. Johnson.

Answer 2

First, we need to find the distance (d) that Mr. Johnson traveled to be able to compute Mrs. Johnson's travel time.
To be able to do that, use the formula d = (s)(t).

d = (80miles/hr)(15mins)
d = (80miles/hr)(0.25hr)
d = 20 miles

Now that we know the distance from Cambridge to Newton, we can use this value to calculate the time Mrs. Johnson needs to complete her travel.

Using the formula t = (d)/(s);
t = (20miles)/(100miles/hr); we get:
t = 0.2hr or 12minutes

Mr. Johnson completed his travel in 15 minutes. But Mrs. Johnson leaves 10 minutes after he does. And to add her 12 minutes travel time, that means Mrs. Johnson will arrive in 22minutes.

Mr. Johnson arrives first by 7 minutes.