Quick estimate of an at-the-money straddle

A stock trades at $20$ with volatility $\sigma = 0.4$. Estimate the value of an at-the-money straddle with $T = 0.36$ years to expiry (ignore rates). Give two decimals.

About $3.83$. An ATM call or put is worth roughly $\sigma S\sqrt{T/(2\pi)} \approx 0.4\, S\sigma\sqrt{T}$. The straddle is twice that: $2 \cdot 0.4 \cdot 20 \cdot \sqrt{0.36/(2\pi)} \approx 3.83$.