Draw a plot to make sure that this "open" square is entirely below the line y = 1-x, 

without having common points with the line.



It tells you that if the point (x,y) is randomly chosen in the square, 

then the probability is 1 (or 100%) that the inequality x + y < 1 is true.



ANSWER.  The probability is equal to 1 (or 100%).

Greenestamps is right because the inside of the square is below the line.
All the points are below the line.

[The line passes through the upper right corner of the square, but we 
are not including points on the line or on the square, just inside (not 'on')
the square, and below (not 'on') the line.]

Edwin