N= 28, number of attendants
s=2 number of slices per person
d= 2 numbers of drinks per person
p= number of pizzas
To calculate the number of pizzas you need you have to multiply the number of slices by the number of people attending the party:
p= (s*N)/8
The total number of drinks (Dr) is equal to the number of drinks per person by the number of attendants:
Dr= d*N
The total cost of the party will be the number of pizzas by the cost per pizza, plus the number of drinks plus the cost per drink:
T= p*$per pizza + Dr*$price per drink.
Number of pizzas: p= (s*N)/8= (2*28)/8= 56/8=7
Assuming they are divided in 8 slices.
Dr= d*N= 2*28= 56 drinks
T= 7*$per pizza + 56*$price per drink.