How to divide a vector into small groups

I have a vector X of size 30 by 1 and I need to divide/group the elements (say the first two forms the first group third and fourth the second group and so on). Now I need to specifically check to which group a particular number (which is an element of X) belongs to and would like to get the index of that number (index when the number belongs to X, say for eg, it was the 15th number in X). Could anyone please help me sort this out.

Answer 1

[David Hill]

x=randi(100,30,1);
X=reshape(x,2,[]);
[~,idxGroup]=find(X==x(15));

[Gopika R]

Thanks a lot. It worked.