Run-of-the-Mills provides your marketing firm with the following data: When the price of guppy gummies decreases by 5%, the quantity of frizzles sold increases by 4% and the quantity of mookies sold decreases by 6%

Two goods are complements if the cross price elasticity is negative. In this example, when the price of guppy gummies decreases by 5%, quantity of fizzles increase by 4%, so the cross price elasticity between guppy gummies and frizzles = -4%/5% = -0.8. So guppy gummies and frizzles are complements.

In contrast, when the price of guppy gummies decreases by 5%, quantity of mookies decreases by 6%, so the cross price elasticity between guppy gummies and frizzles = 6%/5% = 1.2. In this case, mookies and guppy gummies are substitutes.

You should advertise goods that are complements together in a commercial because if the commercial works, consumers are likely to buy more of both. So in this example, you should advertise guppy gummies and mookies together.