With that particular larcenous scheme in mind, he will price the book
regulaly at $50 . See the price-tag in the attached picture.
With a regular price of $50 written all over it, he can run a spectacular,
once-in-a-blue-moon, "40%-off" sale.
40% of $50 = 0.4 x $50 = $20 off
$20 off of $50 ===> Sale price = $30 is what the buyer will pay
during the spectacular sale.
Then, the sale price of $30 is (30/20) = 1.50 of his own original cost.
1.50 of his own cost means 0.50 = 50% more than he paid for it.
