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Answer = 49.

Given Formula = $f$ = $(p + q' + r' + s) \rightarrow (t + u')$

The Truth table for this expression will have $2^6 = 64$ rows (because there are 6 boolean variables) and truth value of $f$ for some of these rows will be $1(True)$ and for others $0(False)$. If you see the expression, It is an Implication expression. And We know that any implication expression $A \rightarrow B$ will be false if and only if $A$ is True and $B$ is False. Otherwise True.

So, Apply same logic here also.

The Given expression will be False iff $(p + q' + r' + s)$ is True and $(t + u')$ is False.

Now, $(t + u')$ is False only when $t = False$ and $u = True$

And $(p + q' + r' + s)$ is always True except when $p = 0, q = 1, r = 1 \,\,and\,\,s = 0$ ..So $(p + q' + r' + s)$ is True in $2^4 -1 = 15$ combinations. 

So, the given expression $f$ = $(p + q' + r' + s) \rightarrow (t + u')$  will be True in $64 - 15 = 49$ rows.

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