A big-screen TV that previously sold for $2,400 has been marked
down to $2,040. What is the percent of decrease?

the percent of the difference is the rate of the difference * 100%

the percent of the difference = .15 * 100% = 15%

since the new price is less than the old price, that is the percent of the decrease.

for finding the midpoint of any given 2 pair of points simply add the respective x coordinates with other x coordinate and the y coordinate with other y coordinate and divide them by 2

in your question it will be :

(3+1)/2 for the x coordinate of mid point i.e 2

[8+(-4)]/2 for the y coordinate of midpoint i.e 2

so coordinates of mid point of points (3,8) and (1,-4) will be (2,2)

the correct answer is the last option