19. Let n be the number of $5 notes.

2(n + 7) + 5n = 182
2n + 14 + 5n = 182
7n + 14 = 182
7n = 168
n = 24 #  or Amount of 24 $5 notes = 24*5 = $120#

18.

Let D be the number of ducks.
Let G be the number of goats.

D – G = 8 … (1)

2D + 4G = 142 … (2)

(1)x4: 4D – 4G = 32 … (3)

(2)+(3): 6D = 174
=> D = 29 #

Let C be the number of chickens.
Let H be the number of horses.

C + H = 54 … (1)

2C + 4H = 138 … (2)

(1)x4: 4C + 4H = 216  … (3)

(3)-(2): 2C = 78
=> C = 39 #

16.

Let n be the number of cupcakes and the number of chicken pies.

3n + 7n = 280
10n = 280
n = 28 #