A count is B(20, 0.15). Find its mean and its standard deviation…

Step 1. The mean. $20 \times 0.15 = 3$.

Step 2. The spread. $\sqrt{20 \times 0.15 \times 0.85} = \sqrt{2.55} \approx 1.597$.

Step 3. The usual range. $3 + 2 \times 1.597 \approx 6.19$. The bottom, $3 - 3.19$, is below $0$, so the range runs from $0$ to about $6$.