When you are determining The probability of an event why must the probability be between 0 and 1

The probability of an event cannot be less than 0 because 0 means it's impossible. The probability of an event cannot be more than 1 because 1 means that it's certain that it will happen. That's why the probability must be between 0 and 1.

area of part 1:

a1=base*height/2=(8+2)*(8-2-2-1)/2=10*3/2=15 ft^2

area of part 2:

a2=length*width=(8+2)*1=10 ft2

area of part 3:

a3=length*width=8*2=16 ft2

total shaded area : a1+a2+a3=41 ft2

total area of the reactangle : a=16*8=128 ft2

total area of the nonshaded region : 128-41=87 ft2