X is going to represent the number of sodas you want to purchase, Y is going to represent the number of popcorn boxes you want to represent, and T is going to represent the total amount of money. So if you want to know how much it is going to cost to purchase all your friends a soda, you would write it as:
X($1.50)=T
You would subsitute how many friends you have for X. For instance, if you wanted to by 5 sodas, you would write it as:
5($1.50)=$7.50
If you want to know how much it is going to cost to purchase all your friends popcorn boxes, you would write it as:
Y($2.50)=T
You would subsitute how many friends you have for Y. For instance, if you wanted to by 5 popcorn boxes, you would write it as:
5($2.50)=$12.50
To determine how much you would need for both, you would simply write out both equations and add them together. For instance, if you wanted to by 5 sodas and 5 popcorn boxes, it would be expressed as:
X($1.50)+Y($2.50)=T
5($1.50)+5($2.50)=T
$7.50+$12.50=T
T=$20.00
Simply put, the answer would be:
X($1.50)+Y($2.50)=T