Probability of number of people who know a rumor

For the expected value/mean, we can ignore the people who haven't heard the rumor entirely. You should find that $M=pK$, which makes sense as the people who haven't heard of the rumor won't add anything to the number of people who tell you they have heard the rumor. This means that $K\approx\frac M p$. Unfortunately, this is only the mean, and not a distribution. Now, what we are going to do is rewrite $M$ as a Binomial Distribution, so $P(X=M)={K\choose M}\cdot p^M\cdot(1-p)^{K-M}$, where $M$ is the number of people who answer yes and $K$ is the total number of people who know the rumor. Now, we want to fix $M$ and let $K$ vary. First, we know that $M\leq K$, so $P(K=x)=0\quad(\forall x<M)$. For $x\geq M$, we can use $$P(X=K|M=m)=\frac{{K\choose M}\cdot p^M\cdot(1-p)^{K-M}}{\int_M^\infty {K\choose M}\cdot p^M\cdot(1-p)^{K-M} dK}$$ Simplifying by considering $p$ and $M$ are constants, we get $$\frac{{K\choose M}\cdot(1-p)^K}{\int_M^\infty{X\choose M}\cdot(1-p)^X dX}$$ We can make one last deduction to simplify this, namely $K\leq n$. This means that we can replace the $\infty$ with an $n$ to get $$\frac{{K\choose M}\cdot(1-p)^K}{\int_M^n{X\choose M}\cdot(1-p)^X dX}$$ I made it continuous, but you can make it discrete by replacing the integral with a summation. $$\frac{{K\choose M}\cdot(1-p)^K}{\sum_{X=M}^n{X\choose M}\cdot(1-p)^X}$$

In summary, I modeled the probability that you got $M$ people telling you if $K$ people knew and if they had a probability of $p$ of telling you using the binomial distribution. I then assumed that $M\leq K \leq n$ and had a uniform distribution because no distribution was specified. I then fixed $M$ since it was known and let $K$ vary. I then found the probability that $M$ out of $K$ people who knew the rumor told you and normalized it across all possible $K$s.