Let r = r1 + ... + rk, where all ri are positive integers. Argue that if X1,... ,Xr has a...

Let r = r1 + ... + rk, where all ri are positive integers. Argue that if X1,... ,Xr has a multinomial distribution, then so does Y1,..., Yk where, with r0 = 0,

That is, Y1 is the sum of the first r1 of the X's, Y2 is the sum of the next r2, and so on.