Three friends split the cost of pizza and a salad. The pizza costs $15 and the salad costs $6.

Each pays $7. 15+6=21 and 21/3=7

Each person would have to pay $7

Split the price of the pizza which is $15, 15 divides by 3 is $5

Then add the $5 per person for the pizza for the cost of the salad divided by 3, $6 divided by three is 2

$2+$5 = $7 per person

Or you could add the cost of the pizza and the salad

$15+$6=$21

And divide that by the number of people

$21 divided by three is $7