as the secret no. is a sq no betn 100 and 1000
then it can be 121,144,169,...,312
it is given that its cube root is an integer,
so it can be 125, 216, 343, 512, 729
among these the nos that are common are:
729.
It suffices to see that cube root of 729 is 9 which is also a square no.
So his no. is 729