We need on #1 we don’t know where the 150 came from

Hi, 150 came from the way they multiplied it, the organization of multiplying, let me demonstrate:
15 * 12 = 
you first multiply out by 2 and get      30then you multiply out by 1 but you have to indent your numbers so:

         30   
    + 15   
     
      180

They just added 0 to the last place after 5 to keep things in order, that 0 doesn't have to be there but you can add it.