Solution
= Solution
Count incidences of a treatment with a block containing it. Summing over blocks gives $bk$, while summing over treatments gives $tr$. Hence the <balanced incomplete block design> satisfies
$$
\boxed{bk=tr.}
$$
= Solution
Count incidences of a treatment with a block containing it. Summing over blocks gives $bk$, while summing over treatments gives $tr$. Hence the <balanced incomplete block design> satisfies
$$
\boxed{bk=tr.}
$$