Ads that depend on poor math skills

The Weather Channel shows me a little weather report whenever I turn on my computer at home. It's nice and useful and funded by ads, such as the following one from Lowe's: "Get $70 off instantly. $5 off purchases of 1 gallon of Valspar paint or $20 off 5 gallons."

Can't I just make 5 individual purchases of 1 gallon each and save $25 off 5 gallons? To save $70, should I make 14 individual purchases of 1 gallon each to get that $70 off, or should I buy 15 gallons and save $60, and then buy two more to save $70? If I bought those 17 gallons individually, would I really save $85? Did anybody read this ad before it was published? Or does a gallon cost $5, or 5 for $20? That would be a nice savings.