Question 924020
Look at the possible outcomes.
HHH - 3 heads - {{{P=(2*2*2)/(3^3)=8/27}}}
HHT - 2 heads - {{{P=(2*2*1)/(3^3)=4/27}}}
HTH - 2 heads - {{{P=(2*1*2)/(3^3)=4/27}}}
HTT - 1 heads - {{{P=(2*1*1)/(3^3)=2/27}}}
THH - 2 heads - {{{P=(1*2*2)/(3^3)=4/27}}}
THT - 1 heads - {{{P=(1*2*1)/(3^3)=2/27}}}
TTH - 1 heads - {{{P=(1*1*2)/(3^3)=2/27}}}
TTT - 0 heads - {{{P=(1*1*1)/(3^3)=1/27}}}
a) {{{P(TTT)=1/27}}}
b) {{{P= (8+4+4+4)/27=20/27}}}