Respuesta :

[tex]20-x=15\\-x=15-20\\-x=-5\\x=5[/tex]

If x = 5 then 3x = 15 because 3(5) = 15 substitute x = 5.

Answer: 15

Step-by-step explanation: 20-x=15

-x=-5

x=5

3x = 3(5)

3x = 15