Explanation
Add up the P(x) values for x = 1, x = 2, and x = 3.
1/2+1/8+1/4 = 4/8+1/8+2/8 = (4+1+2)/8 = 7/8
Another approach would be to notice that P(x) = 1/8 when x = 0, so 1 minus this probability will get us the same answer.
1-(1/8) = 8/8 - 1/8 = (8-1)/8 = 7/8
This is because either x = 0 or x ≥ 1. The two events are complementary.