John sells plain cakes for $8 and decorated cakes for $12. on a particular day, john started with a total of 100 cakes, and sold all but 4. his sales that day totaled $800.
he sold
plain cakes and
decorated cakes that day.

x+ y =  96   →     y = 96 - x     (1)

And the price per plain cake * the number he sold + price of the decorated cake * the number he sold = total sales

So

8x + 12y = 800    (2)

Subbing (1) into (2), we have that

8x + 12 (96 - x)  = 800   simplify

8x + 1152 - 12x  = 800       

-4x + 1152 = 800    subtract 1152 from both sides

-4x  = -352     divide both sides by -4

x = 88   = the number of plain cakes he sold

And 96 - x =  96 - 88  = 8   = the number of decorated cakes he sold