12/4= x/5
Multiply both sides of the equation by 20, the lowest common denominator of 4,5.
5×12=4x
Multiply 5 and 12 to get 60.
60=4x
Swap the sides so that all the terms of the variables are on the left side.
4x=60
Divide both sides by 4.
x = 60/4
Divide 60 by 4 to get 15.
x=15