Here it is given that P(X = 0) = P(Y = 0) = P(X =1) = P(Y=1) = 0.5.
To find : P(X+Y <= 1) so it means X+Y = 0 or X+Y =1 is only possible
P(X+Y =0) = P(X=0) and P(Y= 0) = 0.5 * 0.5 = 0.25.
P(X+Y =1) = P(X=1) and P(Y= 0) OR P(X=0) and P(Y= 1) = 0.5 * 0.5 + 0.5 * 0.5= 0.25 + 0.25 =0.5
so final answer we will get, 0.5 + 0.25 = 0.75