Distance $ = 100$m
Time taken by $A$ to complete $100$m $= t$sec
Time taken by $B$ to complete $100$m $=(t+6)$sec
Time taken by $C$ to complete $100$m $=(t+6)+3=(t+9)$sec
So $A$ beat $C$ by $9$ sec and by $25$m
it means $C$ takes $9$ sec to cover $25$m
Speed of $C$ $=\frac{25}{9}$ m/s
Time taken by $C$ $= \frac{100}{\frac{25}{9}} = 36$ sec
$t+9 =36$ sec
Time take by $A$ to complete $100$m, $t=27$ sec