The performance of candidates in this question was quite good.  The probability that Isaka will be alive while Ashatu will die before the next 20 years is given by
     1  x      {1 – 1  }   =   2     =    1
     6          {      3 }        18          9

The probability that atleast one of them will be alive in 8 years time
    =   1   x   1   +   1   x   2   +  5   x   1     =   4
          6       3         6       3        6        3          9

The probability of obtaining a head in a toss of six fair coins  is
      6C1 (½ )(½)5     =      3   
                                       32.
Probability of atleast four heads  =  Prob. of 4 or 5 or 6 heads
   6C4(½)4 (½)2  + 6C5(½)5 (½) +  6C6(½)6 (½)0   =  15   +  6  x  1    =   11  
                                                                                   64      64    64        32