Question 424673
P(A or B)= P(A) + P(B) - P(A and B)
0.8=0.1+0.9-x

Since x is on the right-hand side of the equation, switch the sides so it is on the left-hand side of the equation.
0.1+0.9-x=0.8

Add 0.9 to 0.1 to get 1.
1-x=0.8

Since 1 does not contain the variable to solve for, move it to the right-hand side of the equation by subtracting 1 from both sides.
-x=-1+0.8

Add 0.8 to -1 to get -0.2.
-x=-0.2

Multiply each term in the equation by -1.
-x*-1=-0.2*-1

Multiply -x by -1 to get x.
x=-0.2*-1

Multiply -0.2 by -1 to get 0.2.
x=0.2