= P(0 heads) + P(2 heads) + P(3 heads) note: Notice that this is is basically the probability that we WILL NOT have 1 head, so it can all be replaced with P(not 1 head).
= P(not 1 head)
= 1 - P(1 head) note: Notice that P(not 1 head) is the same as 1 - P(1 head) or 100% - P(1 head).
= 1 - [(1/2)(1/2)(1/2) + (1/2)(1/2)(1/2) + (1/2)(1/2)(1/2)]
= 1 - [3/8] note: I skipped steps since we calculated P(1 head) in part (a).
= 5/8
If you need any more help, please ask.