Solution
= Solution
The first $m$ letters of the blocked source contain exactly the first $mN$ letters of the original source, so
$$
H(Y_1,\ldots,Y_m)=H(X_1,\ldots,X_{mN}).
$$
If the original <information rate> is $h$, then
$$
\lim_{m\to\infty}\frac1mH(Y_1,\ldots,Y_m)
=N\lim_{m\to\infty}\frac1{mN}H(X_1,\ldots,X_{mN})
=Nh.
$$
\b[Thus fixed-length blocking multiplies information rate by $N$, exactly as recorded by <information rate under fixed-length blocking>.]