The area of a rectangular plot 24 feet long and 16 feet wide will be doubled by adding an equal distance to each side of the plot. What is the distance added to each side?

24*16 = 384

384*2 = 768

24+d * 16+d = 768

384 + 40d+d^2 = 768

d^2 + 40d-384 = 0

(d+48) (d-8) = 0

d=-48, d=8 can't use a negative number so d = 8

check:

24+8=32, 16+8=24, 32x 24 = 768

so 8 feet is added to each side