Answer:
9 pizzas
Step-by-step explanation:
Maria is planning a party. She should order 3 pizzas for every 7 people. Let x be the number of pizzas Maria will order for 21 people, then
3 pizzas - 7 people
x pizzas - 21 people
Write a proportion:
[tex]\dfrac{3}{x}=\dfrac{7}{21}[/tex]
Cross multiply:
[tex]7\cdot x=21\cdot 3\\ \\7x=63\\ \\x=\dfrac{63}{7}=9\ pizzas[/tex]
