# How do you write an equation of a line if it take Andrew 20 minutes to get from a depth of 400 feet to a depth of 50 feet?

##### 1 Answer

We first have to translate to

We can take time as the

If we say Andrew started at time zero at a depth of 400 feet, this translates in a point

A straight line equation is of the form

First we determine

The change is from

change in

change in

(Remember: change is always calculated as new minus old)

Then divide the two:

In the original equation

Time 0, depth 400

So the total equation becomes:

Just to make sure you can also check if this is valid for the other point we know and see what happens if we fill in for

**Challenge:**

After how many minutes does Andrew reach the surface?

(tip: solve for