Ahmed is taking orders for lunch. A slice of pizza costs $1.50 and a chicken sandwich costs $2.50. He collects $21.50 for the group of 9 people. Assuming each person orders only one item, how many people ordered a chicken sandwich

Let number of people who ordered pizza are x and number of people who ordered sandwich are y.

Total number of people = 9, that is x+y=9

Total collection by the 9 people=$21.50, that is

1.50x + 2.50y=21.50.

So we have two equations, which are

x+y=9, 1.50x + 2.50y = 21.50

Solving them, we will get

x=1, y=8

So number of people ordered chicken sandwich = 8.