Answer:
33 1/3%
Step-by-step explanation:
When you buy 20 ads, the per-ad rate is
$1200/20 = $60 . . . per ad
When you buy 80 ads, the per-ad rate is
$3200/80 = $40 . . . per ad
The percentage change to the $60 price is ...
((new price) - (original price))/(original price) × 100%
= ($40 -$60)/$60 × 100% = -20/60 × 100%
= -1/3 × 100% = -33 1/3%
The higher volume purchase comes with a 33 1/3% discount.
