Two numbers, x and y, are randomly chosen from the interval [0,1]. what is the expected value of (x+y)?

It could be  0 and 0 which yeilds a value of 0
it could be 1 and 1 which yields a value of 2

any number from 0 to 2

The expression with an interval of [0,1] means all set of numbers including 0 and 1 and numbers within the range. In this case, the minimum sum could be 0 since x and y could be zero. The maximum is 2 since the two variables could be 1. The interval of x+y should be [0 , 2 ].