MATLAB: How to sum first 140 elements every time in an (1 * 21000) array

Question

a=rand(1,21000);

how to add (1 to 140,141 to 280, 281 to 420……..elements up to 20861 to 21000. and totally i have 21000/140=150 i.e (1row * 150 columns)

and store it in same 'a'

Answer 1

b=sum(reshape(a,[140,150]));