The total cost of 7 windbreakers and 7 blouses was $329. Tina bought 9 such windbreakers, 7 such blouses and a cardigan for $409. If the total cost of a dress and a windbreaker was $51, find the difference in cost between the windbreaker and the dress.
Let 1 W be 1 windbreaker.
Let 1 B be 1 blouse.
Let 1 D be 1 dress.

7 W + 7 B = 329 --- (1)

1 W + 1 B = 329 ÷ 7
1 W + 1 B = 47 --- (2)

1 W + 1 D = 51 --- (3)

9 W + 7 B + 1 D = 409 --- (4)

(4) - (1)
(9 W + 7 B + 1 D) - (7 W + 7 B) = 409 - 329 
9 W - 7 W + 7 B - 7 B + 1 D = 80

2 W + 1 D = 80 --- (5)

(5) - (3)
(2 W + 1 D) - (1 W + 1 D) = 80 - 51
2 W - 1 W + 1 D - 1 D = 29
2 W - 1 W = 29

1 W = 29

From (3)
1 W + 1 D = 51
29 + 1 D = 51

1 D = 51 - 29 = 22

Difference in the cost between the windbreaker and the dress
= 1 W - 1 D
= 29 - 22
= $7

Answer(s): $7