Question

Eight runners run an equal part in a 5-mile relay

Answer 1

Just divide 5 by 8
Use a calculator to check if you need.

5 divided by 8 is 0.625

So, each runner runs 0.625 miles (if that is what you are asking)

Answer 2

So,

I'm assuming you want to find the distance each runner runs.

There are 5 miles to divide among 8 runners.  This is a simple division problem: divide 5 by 8.

$\frac{5}{8}$

You can probably mentally compute the decimal equivalent: 4/8 = 1/2 = 0.5, 1/8 = 0.125, so 0.5 + 0.125 = 0.625.

Each runner will run $\frac{5}{8}$ miles, or 0.625 miles.