Answer
a)256
b)56
c)219
d)70
Work Step by Step
a) Total possible outcomes = 2$\times$2...$\times$2 (8 times) = $2^{8}$ = 256.
b) Exactly three heads = $C$(8,3) = 56
c) At least three heads = $C$(8,3) + $C$(8,4) + $C$(8,5) + $C$(8,6) + $C$(8,7) + $C$(8,8) = 56 + 70 + 56 + 28 + 8 + 1 = 219
d) Equal number of heads and tails i.e. exactly four heads = $C$(8,4) = 70