∵ x is the price of each mug
∵ y is the price of each vase
∵ On Saturday, he sold 3 mugs and 5 vases for a total of $90
∴ 3x + 5y = 90 (first equation)
∵ On the next day, he sold 2 mugs and 4 vases for $70
∴ 2x + 4y = 70 (second equation)
The system of equ