Respuesta :

jcherry99

Answer:

Step-by-step explanation:

You need to set up a proportion.

(21 - 15) / 21  = 4/(x + 4)

6 / 21 = 4/(x + 4)

2/7 = 4/(x + 4)

2(x + 4) = 7*4

2x + 8 = 28

2x + 8-8= 28 - 8

2x = 20

x = 20/2

x = 10

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