Answer:
mean - 12.5
SD - 12.0
Step-by-step explanation:
We substitute the given into the equation
given: a=-1, b=2, c=

=(

-1
=

-1
=

-1
=

-1
to continue, we can either find a common denominator or change the fraction to decimal.
Fraction to Decimal:

=-2.5
Equation=-2.5-1=-3.5
OR Common Denominator:
=


×

(denominator should be -2=common)
=

+

=

which also = to -3.5
Using proportions, it is found that the speed of the runners are of 16 mph and of 20 mph.
<h3>What is a proportion?</h3>
A proportion is a fraction of a total amount.
After 30 minutes, the runners are 2 miles apart, hence the second runner is 4 miles per hour faster than the second, that is:
.
The first runner's speed is 4/5 of the speed of the second runner, hence:





Hence, the speed of the runners are of 16 mph and of 20 mph.
More can be learned about proportions at brainly.com/question/24863377