How to find the number of trials given the success probability and variance?

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If X has a binomial distribution how do you find the number of trials given a success probability and the variance? I'm lost. Could I just divide the variance by the success probability?

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The variance is $\sigma^2=np(1-p)$, so substitute $p$ and $\sigma^2$ into this and rearrange for $n$, i.e. $$n=\frac{\sigma^2}{p(1-p)}.$$