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a. 7j + 14

b. -8x + 24

c. -4 + c

d. -11 - 2b

x = friends that paid discount price = 8

y = friends that paid regular price = 4

Step-by-step explanation:

Let

x = friends that paid discount price

y = friends that paid regular price

x + y = 12 (1)

6x + 8y = 80 (2)

From (1)

x = 12 - y

Substitute x = 12 - y into (2)

6x + 8y = 80 (2)

6(12 - y) + 8y = 80

72 - 6y + 8y = 80

- 6y + 8y = 80 - 72

2y = 8

y = 8/2

y = 4

Substitute y = 4 into (1)

x + y = 12 (1)

x + 4 = 12

x = 12 - 4

x = 8

x = friends that paid discount price = 8

y = friends that paid regular price = 4