Complements, trees and two-event outcomes

Answer: 53​

1. Complement probabilities add to 1:1−104​=53​.

Answer: 21​

1. The four equally likely outcomes are HH, HT, TH and TT. HT and TH each contain exactly one head, giving 42​=21​.

Answer: Multiply the two branch probabilities

1. For the single path H then T, multiply 21​×21​=41​. Add probabilities only when combining distinct paths.