Anyone getting Calc Stack steps figured out?

I’m not so good with math and looking for a little help getting calculations started.

An 8x10 print is $50, but if a client buys additional copies, it’s 30% off, which is $35. I can figure out the percentage without adding it into the equation to save a step?

Client chooses color print 8x10 (quantity 4):
$50 + [($50 - $15 = $35) x 3 = $105) + $50]= $155 final total, that would then be added to any other products chosen to purchase. Some on other pages.

Doable? How many steps would that be? Anything I can do to calculate in my head to make less steps?

Starter file… https://www.dropbox.com/s/v7pootryufpya46/calc%20products.rw.zip?dl=0

@willwood