Once cross multiplied you end up with this:
15x+75=20x−30
Next subtract 20x from both sides.
15x+75−20x=20x−30−20x
−5x+75=−30 -----------> Answer
Now subtract 75 from both sides
−5x+75−75=−30−75
−5x=−105
Now divide by -5 on both sides.
-5/-5x = -105/-5
x = 21 --------------> ANSWER
