Question 499008
(a)
There are 6 choices for each digit
{{{ 6*6*6*6 = 1296 }}}
The boy has a {{{ 1/1296 }}} probability of getting
the right combination
(b)
If he knows the number is odd, then the last digit 
cannot be {{{2}}} or {{{6}}}, so there are {{{4}}}
choices for the last digit
{{{ 6*6*6*4 = 864 }}}
There is a {{{ 1/864 }}} probability of randomly 
getting the right odd combination if he knows
the combination is odd
(c)
If numbers can't be repeated
The number of choices are {{{ 6*5*4*3 = 360 }}}
There is a {{{ 1/360 }}} probability of getting the
right combination if he knows digits can't be 
repeated