When you order the burguer you might get 0 toppings, or one, or two and so on.
Zero toppings can be done by choosing the combination of zero of the 8 possible toppings, that is:
[tex]_8C_0=\frac{8!}{0!(8-0)!}=1[/tex]only one way.
All possible combinations of toppings is the sum of all the terms of the form:
[tex]_8C_r[/tex]where r is the number of toppins. This means that r goes from zero to 8. Then all possible combinations of toppings is:
[tex]\begin{gathered} _8C_0+_8C_1+_8C_2+_8C_3+_8C_4+_8C_5+_8C_6+_8C_7+_8C_8 \\ =1+8+28+56+70+56+28+8+1 \\ =256 \end{gathered}[/tex]Therefore the probability of getting zero toppings is:
[tex]P=\frac{1}{256}[/tex]