If Colby can paint a room in three hours and Patrick can paint the same room in six hours how long would it take if they worked together

20p+22p-12p=90
as all of the terms on the left are 'p's then we consider them to be called like terms. When we have like terms we can add or subtract freely.
so:
20p+22p-12p=90
20p+22p=42p
42p-12p=30p
therefore we get:
30p=90
this is the same as
30*p=90
to find do we need to 'work backwards' to move the 30 to be with 90.
So to 'work backwards' we do the opposite of multiply and divide it instead to both sides of the equation.
30/30*p=90/30
p=90/30
p= 3