There are 12 red and blue pencils on a desk. twice as many red as blue.
how many red pencils on the desk?

There are 12 pencils. 
Red pencils + Blue pencils = 12 

There are twice as many red pencils as blue pencils 
Red pencils = Blue x 2 

Substitute Red Pencils in the first question with blue x 2 

So the first equation is 2 x blue + blue = 12 

This will equal 3 blues = 12 

Divide both sides by 3 

Blues = 4 pencils 

Reds = 2 x blues 

Reds = 2 x 4 

Red = 8 pencils