B = (A >= 20) % same as B = A >= 20, i.e., without parentheses
makes B a matrix of logicals, the same size as A; the elements of B where A is greater than or equal to 20 are true; other elements of B are false.
To get the values of A that are >= 20, you can say:
B = A(A >= 20)
but it won't be a matrix, it will be a column vector. The elements of B will be in the same (column-major) order as they are in A.
Example:
A = [1 3.2 5.4]+[0;1;2;3]
B = A(A >= 4)
Alternatively, if you want B to be a matrix the same size as A, containing the elements of A that are >= 20 located at the same locations where they are in A and say zeros or NaNs everywhere else, you can do something like this:
B = NaN(size(A)); % or B = zeros(size(A)); % for instance
idx = A >= 4;
B(idx) = A(idx)
