I came up with the following equation to solve: 120x=(x+10)(60y-4x)
Whole numbers don't work, possible solution is
x=10
y=5/3
Old price 1/6$, for a dozen this makes 2$
Nieuw price 0.1$, for a dozen this makes 1.2$
EDIT: x=5, y=1 work as well and are actually whole numbers. R00fles!