60 items were for sale because you need to add together the amounts that were sold plus what was left over, giving you everything that they had for sale.
Multiplying both sides of the equation by 3 and substituting 4 for p
Multiple 6 with numbers inside paranthesis:
12x - 66 + 15 = 21
add 66 fhen substrack 15 from both sides :
12x = 72 divide both sides with 12:
x = 6