lets take size of hash table is K
and no of entries filled in hash table =n ;so probability of collision = n/k
if we insert 't' more entries in the hash table . so now total filled entries =n+t ; so now probability of collision =(n+t)/k
as by the question (n+t)/k - (n/k) =0.6 ; and k =15
so t/k=0.6 => t=15*0.6 =9
so after hashing 9 more elements probability of colision will increase by 0.6