The HT outcome is twice as likely (as the HH and TT ones) to occur, since it can occur as HT or TH, which makes for four events of equal probability. Thus $$E=\frac{0+2×12+60}4=21$$
The HT outcome is twice as likely (as the HH and TT ones) to occur, since it can occur as HT or TH, which makes for four events of equal probability. Thus $$E=\frac{0+2×12+60}4=21$$