It costs a family $324 to buy a 10-ft-by-12-ft rug. at this rate, what will it cost them to buy a 15-ft-by-18-ft rug?

Area of the rug the family bought = 10 x 12 = 120 ft²

120 ft² cost $234, find the cost of 1 ft²

⇒ 120ft² = $324

⇒ 1ft² = $324 ÷ 120 = $2.70

Area of the new rug = 15 x 18 = 270 ft²

1 ft² = $2.70, find the cost of 270 ft²

⇒ 1 ft² = $2.70

⇒ 270 ft² = 270 x $2.70 = $729

The 15-ft by 18-ft rug will cost $729