A store sells the basic model of a phone for $60 and the upgraded model for $200. It costs the store $10 to buy each basic model
and $50 to buy each upgraded model from the phone company. On Tuesday, the store has $1,800 in sales for these two models,
and it paid the phone company $400 to buy these phones. How many upgraded models did the store sell on Tuesday?
Let x = number of basic models.
Let y = number of upgraded models.
Sales: 60x + 200 y = 1800 60x + 200y = 1800
Purchase: 10x + 50y = 400 ---> x -4 ---> -40x - 200y = -1600
Adding down the columns: 20x = 200
x = 10 <--- number of basic models
Substituting: 10x + 50y = 400 ---> 10(10) + 50y = 400 ---> 100 + 50y = 400
---> 50y = 300 ---> y = 6 <--- number of upgraded models
Let x = number of basic models.
Let y = number of upgraded models.
Sales: 60x + 200 y = 1800 60x + 200y = 1800
Purchase: 10x + 50y = 400 ---> x -4 ---> -40x - 200y = -1600
Adding down the columns: 20x = 200
x = 10 <--- number of basic models
Substituting: 10x + 50y = 400 ---> 10(10) + 50y = 400 ---> 100 + 50y = 400
---> 50y = 300 ---> y = 6 <--- number of upgraded models