Probability of H=2/3, probability of T=1/3.
All 16 outcomes with their probabilities:
HHHH=16/81
HHHT, HHTH, HTHH, THHH=8/81
HHTT,HTHT,HTTH,THHT,THTH,TTHH=4/81
HTTT,THTT,TTHT,TTTH=2/81
TTTT=1/81
a) Probability of exactly 2 tails: 6×4/81=24/81=8/27.
b) Probability of at least 3 heads: 4×8/81+16/81=48/81=16/27.