Burger and sandwich

Using the general binomial probability formula,

$P$ ( $k$ out of $n$ ) = $\dfrac{n!}{k!(n - k)!}$ $p^k$ $(1 - p)^{(n - k)}$

Substitute ,

$p = 0.4$

$n = 7$

$k = 4$

Now,

$P$ ( $4$ Burgers out of $7$ ) = $\dfrac{7!}{4!(7 - 4)!}$ $(0.4)^4$ $(1 - 0.4)^{(7 - 4)}$

$=$ $\dfrac{7!}{4!3!}$ $(0.4)^4$ $(0.6)^3$

$=$ $\dfrac{7 × 6 × 5 × 4 × 3 × 2 × 1}{4 × 3 × 2 × 1 × 3 × 2 × 1 }$ $(0.4)^4$ $(0.6)^3$

$= 35 × 0.0256 × 0.216$

$= 0.193536$

$= 0.19$ is the answer with $2$ decimal places